Add Blink flag and IDLs for Delegated Ink Trail

Create a Blink flag to hide upcoming feature behind, and add skeleton
IDLs, .cc and .h files for the feature.

TL;DR of feature: Provide developers a simple JS API to improve inking
latency. Latency is reduced by forwarding points from browser to GPU
process more directly, and then drawing them on the swapchain just
before the swap occurs, connecting them to the point most recently
rendered by the app.

Explainer here: https://github.com/WICG/ink-enhancement
i2p: https://groups.google.com/a/chromium.org/d/msg/blink-dev/JeGBzAfR_Bw/Ppo1u7ofBgAJ

Bug: 1052145
Change-Id: Ib0e1b1f6c4fbeaee551ca37770dbd72bdeeaa6b1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2161932
Reviewed-by: Navid Zolghadr <[email protected]>
Reviewed-by: Daniel Libby <[email protected]>
Reviewed-by: Kentaro Hara <[email protected]>
Commit-Queue: Mario Bianucci <[email protected]>
Cr-Commit-Position: refs/heads/master@{#763903}

delegated-ink/requestPresenter-returns-valid-promise.tentative.window.js

@@ -0,0 +1,11 @@
+test(() => {
+  assert_implements(typeof navigator.ink !== "undefined", 'ink is not supported');
+}, "navigator needs to support ink to run this test.");
+
+promise_test(t => {
+  return promise_rejects_js(t, TypeError, navigator.ink.requestPresenter('bad-type'));
+}, "Receive rejected promise for a bad type.");
+
+promise_test(() => {
+  return navigator.ink.requestPresenter('delegated-ink-trail');
+}, "Received fulfilled promise for a good type.");