[wptrunner] Enable print-reftests for content shell (#36471)

This is done by passing the 'print specifier over stdin.

Tested with:
  xvfb-run wpt run ... infrastructure/reftest/*-print.html \
    --metadata=infrastructure/metadata

All except `reftest_mismatch_page_margins-print.html` should pass
(`@page` does not currently work).

tools/wptrunner/wptrunner/browsers/content_shell.py

@@ -10,17 +10,23 @@
 from .base import Browser, ExecutorBrowser
 from .base import get_timeout_multiplier   # noqa: F401
 from ..executors import executor_kwargs as base_executor_kwargs
-from ..executors.executorcontentshell import (ContentShellRefTestExecutor,  # noqa: F401
-                                              ContentShellCrashtestExecutor,  # noqa: F401
-                                              ContentShellTestharnessExecutor)  # noqa: F401
+from ..executors.executorcontentshell import (  # noqa: F401
+    ContentShellCrashtestExecutor,
+    ContentShellPrintRefTestExecutor,
+    ContentShellRefTestExecutor,
+    ContentShellTestharnessExecutor,
+)
 
 
 __wptrunner__ = {"product": "content_shell",
                  "check_args": "check_args",
                  "browser": "ContentShellBrowser",
-                 "executor": {"reftest": "ContentShellRefTestExecutor",
+                 "executor": {
                      "crashtest": "ContentShellCrashtestExecutor",
-                     "testharness": "ContentShellTestharnessExecutor"},
+                     "print-reftest": "ContentShellPrintRefTestExecutor",
+                     "reftest": "ContentShellRefTestExecutor",
+                     "testharness": "ContentShellTestharnessExecutor",
+                 },
                  "browser_kwargs": "browser_kwargs",
                  "executor_kwargs": "executor_kwargs",
                  "env_extras": "env_extras",

tools/wptrunner/wptrunner/executors/executorcontentshell.py

@@ -48,10 +48,14 @@ def __init__(self, parent):
         self.stdout_queue = parent.browser.stdout_queue
         self.stdin_queue = parent.browser.stdin_queue
 
-    def do_test(self, url, timeout=None):
-        """Sends a url to content_shell and returns the resulting text and image output.
+    def do_test(self, command, timeout=None):
+        """Send a command to content_shell and return the resulting outputs.
+
+        A command consists of a URL to navigate to, followed by an optional
+        expected image hash and 'print' mode specifier. The syntax looks like:
+            http://web-platform.test:8000/test.html['<hash>['print]]
         """
-        self._send_command(url)
+        self._send_command(command)
 
         deadline = time() + timeout if timeout else None
         # The first block can also contain audio data but not in WPT.
@@ -201,15 +205,32 @@ def do_test(self, test):
             return _convert_exception(test, exception, self.protocol.content_shell_errors.read_errors())
 
     def screenshot(self, test, viewport_size, dpi, page_ranges):
-        _, image = self.protocol.content_shell_test.do_test(self.test_url(test),
-                test.timeout * self.timeout_multiplier)
+        # Currently, the page size and DPI are hardcoded for print-reftests:
+        #   https://chromium.googlesource.com/chromium/src/+/4e1b7bc33d42b401d7d9ad1dcba72883add3e2af/content/web_test/renderer/test_runner.cc#100
+        # Content shell has an internal `window.testRunner.setPrintingSize(...)`
+        # API, but it's not callable with protocol mode.
+        assert dpi is None
+        command = self.test_url(test)
+        if self.is_print:
+            # Currently, `content_shell` uses the expected image hash to avoid
+            # dumping a matching image as an optimization. In Chromium, the
+            # hash can be computed from an expected screenshot checked into the
+            # source tree (i.e., without looking at a reference). This is not
+            # possible in `wpt`, so pass an empty hash here to force a dump.
+            command += "''print"
+        _, image = self.protocol.content_shell_test.do_test(
+            command, test.timeout * self.timeout_multiplier)
 
         if not image:
             return False, ("ERROR", self.protocol.content_shell_errors.read_errors())
 
         return True, b64encode(image).decode()
 
 
+class ContentShellPrintRefTestExecutor(ContentShellRefTestExecutor):
+    is_print = True
+
+
 class ContentShellCrashtestExecutor(CrashtestExecutor):
     def __init__(self, logger, browser, server_config, timeout_multiplier=1, debug_info=None,
             **kwargs):