Prevent dirty rect calculation every frame

Bug: 1044993
Change-Id: I14c8c09b04aba9c2e81f76792f6f9f9a8e7fbd35
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2017042
Commit-Queue: Fernando Serboncini <fserb@chromium.org>
Reviewed-by: Juanmi Huertas <juanmihd@chromium.org>
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Auto-Submit: Fernando Serboncini <fserb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734564}

third_party/blink/renderer/platform/graphics/canvas_2d_layer_bridge.cc

@@ -641,8 +641,14 @@ void Canvas2DLayerBridge::FlushRecording() {
 
   // Sample one out of every kRasterMetricProbability frames to time
   // If the canvas is accelerated, we also need access to the raster_interface
-  bool measure_raster_metric = (raster_interface || !IsAccelerated()) &&
-                               bernoulli_distribution_(random_generator_);
+
+  // We are using @dont_use_idle_scheduling_for_testing_ temporarily to always
+  // measure while testing.
+  const bool will_measure = dont_use_idle_scheduling_for_testing_ ||
+                            bernoulli_distribution_(random_generator_);
+  const bool measure_raster_metric =
+      (raster_interface || !IsAccelerated()) && will_measure;
+
   RasterTimer rasterTimer;
   base::Optional<base::ElapsedTimer> timer;
   // Start Recording the raster duration
@@ -662,7 +668,9 @@ void Canvas2DLayerBridge::FlushRecording() {
     SkScalar canvas_height = canvas->getLocalClipBounds().height();
     DCHECK_GE(canvas_width, size_.Width());
     DCHECK_GE(canvas_height, size_.Height());
-    CalculateDirtyRegion(canvas_width, canvas_height);
+    if (will_measure) {
+      CalculateDirtyRegion(canvas_width, canvas_height);
+    }
 
     canvas->drawPicture(last_recording_);
     last_record_tainted_by_write_pixels_ = false;