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

include/gpu/GrContext.h

Index: include/gpu/GrContext.h
diff --git a/include/gpu/GrContext.h b/include/gpu/GrContext.h
index 5f964578569b52c4cf1c6b51b34e614e9f37fa9e..d2a3ada9d688a4cafdb42dd80dde2e907319ec9a 100644
--- a/include/gpu/GrContext.h
+++ b/include/gpu/GrContext.h
@@ -653,7 +653,13 @@ public:
      * perform a resolve to a GrTexture used as the source of a draw or before
      * reading pixels back from a GrTexture or GrRenderTarget.
      */
-    void resolveRenderTarget(GrRenderTarget* target);
+    void resolveRenderTarget(GrRenderTarget*);
+
+    /**
+     * Provides a perfomance hint that the render target's contents are allowed
+     * to become undefined.
+     */
+    void discardRenderTarget(GrRenderTarget*);
 
 #ifdef SK_DEVELOPER
     void dumpFontCache() const;