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

src/core/SkCanvas.cpp

Index: src/core/SkCanvas.cpp
diff --git a/src/core/SkCanvas.cpp b/src/core/SkCanvas.cpp
index e8336fffffc8515ba4b24b15d0ec086e11e03eb7..e575aec98b210e813af61f9b11826c1dabf11411 100644
--- a/src/core/SkCanvas.cpp
+++ b/src/core/SkCanvas.cpp
@@ -1912,6 +1912,12 @@ void SkCanvas::clear(SkColor color) {
     }
 }
 
+void SkCanvas::onDiscard() {
+    if (NULL != fSurfaceBase) {
+        fSurfaceBase->aboutToDraw(SkSurface::kDiscard_ContentChangeMode);
+    }
+}
+
 void SkCanvas::drawPaint(const SkPaint& paint) {
     this->internalDrawPaint(paint);
 }