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

src/gpu/gl/GrGLCaps.h

Index: src/gpu/gl/GrGLCaps.h
diff --git a/src/gpu/gl/GrGLCaps.h b/src/gpu/gl/GrGLCaps.h
index 7f0c8874ca2e441245c98aabb21d44ab931e6d10..ade9d5d473b7d2621d870ce0a3a69bf4cd0338de 100644
--- a/src/gpu/gl/GrGLCaps.h
+++ b/src/gpu/gl/GrGLCaps.h
@@ -245,9 +245,6 @@ public:
 
     bool fixedFunctionSupport() const { return fFixedFunctionSupport; }
 
-    /// Is there support for discarding the frame buffer
-    bool discardFBSupport() const { return fDiscardFBSupport; }
-
     bool fullClearIsFree() const { return fFullClearIsFree; }
 
 private:
@@ -330,7 +327,6 @@ private:
     bool fUseNonVBOVertexAndIndexDynamicData : 1;
     bool fIsCoreProfile : 1;
     bool fFixedFunctionSupport : 1;
-    bool fDiscardFBSupport : 1;
     bool fFullClearIsFree : 1;
 
     typedef GrDrawTargetCaps INHERITED;