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

include/gpu/GrRenderTarget.h

Index: include/gpu/GrRenderTarget.h
diff --git a/include/gpu/GrRenderTarget.h b/include/gpu/GrRenderTarget.h
index f43aba9d589eb8811151d582ee5c524a2af0f7ff..ac3cbee054c630541bb77bf8d9883c2e2f4e854d 100644
--- a/include/gpu/GrRenderTarget.h
+++ b/include/gpu/GrRenderTarget.h
@@ -121,6 +121,12 @@ public:
      */
     void resolve();
 
+    /**
+     * Provide a performance hint that the render target's contents are allowed
+     * to become undefined.
+     */
+    void discard();
+
     // a MSAA RT may require explicit resolving , it may auto-resolve (e.g. FBO
     // 0 in GL), or be unresolvable because the client didn't give us the
     // resolve destination.