Fix text missing in performance overlay (on Android) (#49035)

The Android fontmgr does not return a default typeface when a nullptr
style is passed in [1] (I'm not sure if this is a feature or a bug).
However, calling legacyMakeTypeface will use a default sans-serif font.
This adds in that backup behavior.

[1]
https://github.com/google/skia/blob/4170fba2d89fc0319aad5702a15acdee4470f606/src/ports/SkFontMgr_android.cpp#L310-L312

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide] and the [C++,
Objective-C, Java style guides].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I added new tests to check the change I am making or feature I am
adding, or the PR is [test-exempt]. See [testing the engine] for
instructions on writing and running engine tests.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I signed the [CLA].
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#overview
[Tree Hygiene]: https://github.com/flutter/flutter/wiki/Tree-hygiene
[test-exempt]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[C++, Objective-C, Java style guides]:
https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
[testing the engine]:
https://github.com/flutter/flutter/wiki/Testing-the-engine
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#handling-breaking-changes
[Discord]: https://github.com/flutter/flutter/wiki/Chat

flow/layers/performance_overlay_layer.cc

@@ -76,10 +76,19 @@ sk_sp<SkTextBlob> PerformanceOverlayLayer::MakeStatisticsText(
   SkFont font;
   sk_sp<SkFontMgr> font_mgr = txt::GetDefaultFontManager();
   if (font_path == "") {
-    font = SkFont(font_mgr->matchFamilyStyle(nullptr, {}), 15);
+    if (sk_sp<SkTypeface> face = font_mgr->matchFamilyStyle(nullptr, {})) {
+      font = SkFont(face, 15);
+    } else {
+      // In Skia's Android fontmgr, matchFamilyStyle can return null instead
+      // of falling back to a default typeface. If that's the case, we can use
+      // legacyMakeTypeface, which *does* use that default typeface.
+      font = SkFont(font_mgr->legacyMakeTypeface(nullptr, {}), 15);
+    }
   } else {
     font = SkFont(font_mgr->makeFromFile(font_path.c_str()), 15);
   }
+  // Make sure there's not an empty typeface returned, or we won't see any text.
+  FML_DCHECK(font.getTypeface()->countGlyphs() > 0);
 
   double max_ms_per_frame = stopwatch.MaxDelta().ToMillisecondsF();
   double average_ms_per_frame = stopwatch.AverageDelta().ToMillisecondsF();