canvas: clean up start/stop listening for DidProcessTask

Bit of a cleanup accesory to crrev.com/c/1308043: a new pair
of private methods (StartListeningForDidProcessTask() and
StopListeningForDidProcessTask() ) are added. They work together
with the associate renamed flag
s/finalize_frame_scheduled_/listening_for_did_process_task_/
to listen for ProcessTask events (WillProcessTask/DidProcessTask).

This is a cleanup of things in preparation for the next CL
in the chain: crrev.com/c/1308043.

No new functionality intended.

Bug: 839970
Change-Id: I0bad9e3bfc80b02865c34ba4c20d37f71ecb83a9
Reviewed-on: https://chromium-review.googlesource.com/c/1308754Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604912}

third_party/blink/renderer/core/html/canvas/canvas_rendering_context.cc

@@ -99,8 +99,7 @@ WTF::String CanvasRenderingContext::PixelFormatAsString() const {
 }
 
 void CanvasRenderingContext::Dispose() {
-  if (finalize_frame_scheduled_)
-    Platform::Current()->CurrentThread()->RemoveTaskObserver(this);
+  StopListeningForDidProcessTask();
 
   // HTMLCanvasElement and CanvasRenderingContext have a circular reference.
   // When the pair is no longer reachable, their destruction order is non-
@@ -116,25 +115,22 @@ void CanvasRenderingContext::Dispose() {
 
 void CanvasRenderingContext::DidDraw(const SkIRect& dirty_rect) {
   Host()->DidDraw(SkRect::Make(dirty_rect));
-  NeedsFinalizeFrame();
+  StartListeningForDidProcessTask();
 }
 
 void CanvasRenderingContext::DidDraw() {
   Host()->DidDraw();
-  NeedsFinalizeFrame();
+  StartListeningForDidProcessTask();
 }
 
 void CanvasRenderingContext::NeedsFinalizeFrame() {
-  if (!finalize_frame_scheduled_) {
-    finalize_frame_scheduled_ = true;
-    Platform::Current()->CurrentThread()->AddTaskObserver(this);
-  }
+  StartListeningForDidProcessTask();
 }
 
 void CanvasRenderingContext::DidProcessTask(
-    const base::PendingTask& pending_task) {
-  Platform::Current()->CurrentThread()->RemoveTaskObserver(this);
-  finalize_frame_scheduled_ = false;
+    const base::PendingTask& /* pending_task */) {
+  StopListeningForDidProcessTask();
+
   // The end of a script task that drew content to the canvas is the point
   // at which the current frame may be considered complete.
   if (Host())
@@ -202,4 +198,20 @@ void CanvasRenderingContext::Trace(blink::Visitor* visitor) {
   ScriptWrappable::Trace(visitor);
 }
 
+void CanvasRenderingContext::StartListeningForDidProcessTask() {
+  if (listening_for_did_process_task_)
+    return;
+
+  listening_for_did_process_task_ = true;
+  Platform::Current()->CurrentThread()->AddTaskObserver(this);
+}
+
+void CanvasRenderingContext::StopListeningForDidProcessTask() {
+  if (!listening_for_did_process_task_)
+    return;
+
+  Platform::Current()->CurrentThread()->RemoveTaskObserver(this);
+  listening_for_did_process_task_ = false;
+}
+
 }  // namespace blink

third_party/blink/renderer/core/html/canvas/canvas_rendering_context.h

@@ -207,7 +207,10 @@ class CORE_EXPORT CanvasRenderingContext : public ScriptWrappable,
   HashSet<String> dirty_urls_;
   CanvasColorParams color_params_;
   CanvasContextCreationAttributesCore creation_attributes_;
-  bool finalize_frame_scheduled_ = false;
+
+  void StartListeningForDidProcessTask();
+  void StopListeningForDidProcessTask();
+  bool listening_for_did_process_task_ = false;
 
   DISALLOW_COPY_AND_ASSIGN(CanvasRenderingContext);
 };

third_party/blink/renderer/modules/canvas/canvas2d/base_rendering_context_2d.h

@@ -372,7 +372,7 @@ class MODULES_EXPORT BaseRenderingContext2D : public GarbageCollectedMixin,
 
   mutable UsageCounters usage_counters_;
 
-  virtual void NeedsFinalizeFrame(){};
+  virtual void NeedsFinalizeFrame() {}
 
   float GetFontBaseline(const SimpleFontData&) const;