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

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Author: kjlubick

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();