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


BUG=skia:2349
R=robertphillips@google.com, reed@google.com

Author: bsalomon@google.com

Review URL: https://codereview.chromium.org/211683002

git-svn-id: http://skia.googlecode.com/svn/trunk@13976 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/core/SkCanvas.cpp b/src/core/SkCanvas.cpp
index e8336ff..e575aec 100644
--- a/src/core/SkCanvas.cpp
+++ b/src/core/SkCanvas.cpp
@@ -1912,6 +1912,12 @@
     }
 }
 
+void SkCanvas::onDiscard() {
+    if (NULL != fSurfaceBase) {
+        fSurfaceBase->aboutToDraw(SkSurface::kDiscard_ContentChangeMode);
+    }
+}
+
 void SkCanvas::drawPaint(const SkPaint& paint) {
     this->internalDrawPaint(paint);
 }