Treat calls to CanvasRenderingContext2D.clearRect as operations that clear the canvas

Source/platform/graphics/OpaqueRectTrackingContentLayerDelegate.cpp

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
  *     notice, this list of conditions and the following disclaimer in the
@@ skipping 40 matching lines @@
 void OpaqueRectTrackingContentLayerDelegate::paintContents(
     SkCanvas* canvas, const WebRect& clip, bool canPaintLCDText, WebFloatRect& opaque,
     blink::WebContentLayerClient::GraphicsContextStatus contextStatus)
 {
     static const unsigned char* annotationsEnabled = 0;
     if (UNLIKELY(!annotationsEnabled))
         annotationsEnabled = EventTracer::getTraceCategoryEnabledFlag(TRACE_DISABLED_BY_DEFAULT("blink.graphics_context_annotations"));
 
     GraphicsContext context(canvas,
         contextStatus == blink::WebContentLayerClient::GraphicsContextEnabled ? GraphicsContext::NothingDisabled : GraphicsContext::FullyDisabled);
-    context.setTrackOpaqueRegion(!m_opaque);
+    context.setRegionTrackingMode(m_opaque ? GraphicsContext::RegionTrackingDisabled : GraphicsContext::RegionTrackingOpaque);
     context.setCertainlyOpaque(m_opaque);
     context.setShouldSmoothFonts(canPaintLCDText);
 
     if (*annotationsEnabled)
         context.setAnnotationMode(AnnotateAll);
 
     // Record transform prior to painting, as all opaque tracking will be
     // relative to this current value.
     AffineTransform canvasToContentTransform = context.getCTM().inverse();
 
     m_painter->paint(context, clip);
 
     // Transform tracked opaque paints back to our layer's content space.
     ASSERT(canvasToContentTransform.isInvertible());
     ASSERT(canvasToContentTransform.preservesAxisAlignment());
     opaque = canvasToContentTransform.mapRect(context.opaqueRegion().asRect());
 }
 
 }