Add discard API to SkCanvas, plumb it to glDiscardFramebuffer()

src/image/SkSurface_Gpu.cpp

Index: src/image/SkSurface_Gpu.cpp
diff --git a/src/image/SkSurface_Gpu.cpp b/src/image/SkSurface_Gpu.cpp
index 50d6606145a644d9a69be50613555854cc4d6da6..2f32d2fc7ce3d02225b7b5d516af53f9d6325adf 100644
--- a/src/image/SkSurface_Gpu.cpp
+++ b/src/image/SkSurface_Gpu.cpp
@@ -23,6 +23,7 @@ public:
     virtual void onDraw(SkCanvas*, SkScalar x, SkScalar y,
                         const SkPaint*) SK_OVERRIDE;
     virtual void onCopyOnWrite(ContentChangeMode) SK_OVERRIDE;
+    virtual void onDiscard() SK_OVERRIDE;
 
 private:
     SkGpuDevice* fDevice;
@@ -86,9 +87,15 @@ void SkSurface_Gpu::onCopyOnWrite(ContentChangeMode mode) {
 
         this->getCachedCanvas()->setRootDevice(newDevice);
         SkRefCnt_SafeAssign(fDevice, newDevice);
+    } else if (kDiscard_ContentChangeMode == mode) {
+        this->SkSurface_Gpu::onDiscard();
     }
 }
 
+void SkSurface_Gpu::onDiscard() {
+    fDevice->accessRenderTarget()->discard();
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 
 SkSurface* SkSurface::NewRenderTargetDirect(GrRenderTarget* target) {