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

Source/platform/graphics/GraphicsContext.h

Index: Source/platform/graphics/GraphicsContext.h
diff --git a/Source/platform/graphics/GraphicsContext.h b/Source/platform/graphics/GraphicsContext.h
index 3c59c53735b1dc12a23af0e8442bcbdad118148b..87f67004118444c08b5d3fa29588619755861275 100644
--- a/Source/platform/graphics/GraphicsContext.h
+++ b/Source/platform/graphics/GraphicsContext.h
@@ -224,6 +224,7 @@ public:
     // The opaque region is empty until tracking is turned on.
     // It is never clerared by the context.
     void setTrackOpaqueRegion(bool track) { m_trackOpaqueRegion = track; }
+    void setTreatOverwriteAsOpaque(bool value) { m_opaqueRegion.setTreatOverwriteAsOpaque(value); }

[Stephen White, 2014/07/25 19:29:06]

As discussed, let's make this a setter that specifies the mode of the region
tracker, since the difference is subtle but important. Something like:

setTrackRegion({OPAQUE | OVERWRITE | NONE});

     const OpaqueRegionSkia& opaqueRegion() const { return m_opaqueRegion; }
 
     // The text region is empty until tracking is turned on.