Add non-blocking frame rate limiting logic to display list 2D canvas

Source/platform/graphics/RecordingImageBufferSurface.cpp

Index: Source/platform/graphics/RecordingImageBufferSurface.cpp
diff --git a/Source/platform/graphics/RecordingImageBufferSurface.cpp b/Source/platform/graphics/RecordingImageBufferSurface.cpp
index abd7003fa38821b46f8c9c7bdc24eb30e8d309ea..bad61686e84800754194f13b962f6e53b2247699 100644
--- a/Source/platform/graphics/RecordingImageBufferSurface.cpp
+++ b/Source/platform/graphics/RecordingImageBufferSurface.cpp
@@ -8,6 +8,7 @@
 
 #include "platform/graphics/GraphicsContext.h"
 #include "platform/graphics/ImageBuffer.h"
+#include "public/platform/Platform.h"
 #include "third_party/skia/include/core/SkCanvas.h"
 #include "third_party/skia/include/core/SkPictureRecorder.h"
 #include "wtf/PassOwnPtr.h"
@@ -20,13 +21,17 @@ RecordingImageBufferSurface::RecordingImageBufferSurface(const IntSize& size, Op
     , m_graphicsContext(0)
     , m_initialSaveCount(0)
     , m_frameWasCleared(true)
-    , m_surfaceUsedSincePreviousFrameWasPresented(false)
+    , m_recordedSinceLastFrameWasFinalized(false)
 {
     initializeCurrentFrame();
 }
 
 RecordingImageBufferSurface::~RecordingImageBufferSurface()
-{ }
+{
+    if (m_recordedSinceLastFrameWasFinalized) {
+        blink::Platform::current()->currentThread()->removeTaskObserver(this);
+    }
+}
 
 void RecordingImageBufferSurface::initializeCurrentFrame()
 {
@@ -90,8 +95,7 @@ SkCanvas* RecordingImageBufferSurface::canvas() const
 
 PassRefPtr<SkPicture> RecordingImageBufferSurface::getPicture()
 {
-    if (handleOpaqueFrame()) {
-        m_surfaceUsedSincePreviousFrameWasPresented = false;
+    if (finalizeFrame()) {
         return m_previousFrame;
     }
 
@@ -100,9 +104,30 @@ PassRefPtr<SkPicture> RecordingImageBufferSurface::getPicture()
     return nullptr;
 }
 
-void RecordingImageBufferSurface::willUse()
+void RecordingImageBufferSurface::didDraw()
+{
+    if (!m_recordedSinceLastFrameWasFinalized && m_currentFrame) {
+        m_recordedSinceLastFrameWasFinalized = true;
+        blink::Platform::current()->currentThread()->addTaskObserver(this);
+    }
+}
+
+void RecordingImageBufferSurface::willProcessTask()
+{
+}
+
+void RecordingImageBufferSurface::didProcessTask()
 {
-    m_surfaceUsedSincePreviousFrameWasPresented = true;
+    ASSERT(m_recordedSinceLastFrameWasFinalized);
+    // This is to insert a frame barrier at the end of each script execution
+    // task that touches the canvas. finalizeFrame() will discard the previous
+    // frame and replace it with the current frame even if it has not been
+    // consumed, thus allowing the renderer to skip ahead to the most recently
+    // recorded frame.
+    if (!finalizeFrame()) {
+        ASSERT(!m_rasterCanvas);
+        fallBackToRasterCanvas();
+    }
 }
 
 void RecordingImageBufferSurface::didClearCanvas()
@@ -110,14 +135,23 @@ void RecordingImageBufferSurface::didClearCanvas()
     m_frameWasCleared = true;
 }
 
-bool RecordingImageBufferSurface::handleOpaqueFrame()
+bool RecordingImageBufferSurface::finalizeFrame()
 {
-    if (!m_currentFrame)
+    if (!m_currentFrame) {
+        ASSERT(!m_recordedSinceLastFrameWasFinalized);
         return false;
+    }
+
+    bool canvasHasChanged = m_recordedSinceLastFrameWasFinalized;
+    if (m_recordedSinceLastFrameWasFinalized) {
+        blink::Platform::current()->currentThread()->removeTaskObserver(this);
+        m_recordedSinceLastFrameWasFinalized = false;
+    }
+
     IntRect canvasRect(IntPoint(0, 0), size());
     if (!m_frameWasCleared && !m_graphicsContext->opaqueRegion().asRect().contains(canvasRect)) {
         // No clear happened. If absolutely nothing was drawn, then we can just continue to use the previous frame.
-        return !m_surfaceUsedSincePreviousFrameWasPresented;
+        return !canvasHasChanged;
     }
 
     SkCanvas* oldCanvas = m_currentFrame->getRecordingCanvas(); // Could be raster or picture