Implement SkBaseDevice snapshot support

src/utils/SkDeferredCanvas.cpp

Index: src/utils/SkDeferredCanvas.cpp
diff --git a/src/utils/SkDeferredCanvas.cpp b/src/utils/SkDeferredCanvas.cpp
index faf1cf974a0c8682941b6e20259212976e23b75d..1e955b378239a421c1e1968529e518bd9170e415 100644
--- a/src/utils/SkDeferredCanvas.cpp
+++ b/src/utils/SkDeferredCanvas.cpp
@@ -259,7 +259,7 @@ protected:
 
 private:
     void flush() SK_OVERRIDE;
-    void replaceBitmapBackendForRasterSurface(const SkBitmap&) SK_OVERRIDE {}
+    void replaceBitmapBackend(const SkBitmap&) SK_OVERRIDE {}
 
     void beginRecording();
     void init();
@@ -353,9 +353,7 @@ void SkDeferredDevice::aboutToDraw()
         fNotificationClient->prepareForDraw();
     }
     if (fCanDiscardCanvasContents) {
-        if (fSurface) {
-            fSurface->notifyContentWillChange(SkSurface::kDiscard_ContentChangeMode);
-        }
+        fImmediateCanvas->discard();
         fCanDiscardCanvasContents = false;
     }
 }
@@ -444,11 +442,7 @@ void SkDeferredDevice::prepareForImmediatePixelWrite() {
     if (fPipeController.hasPendingCommands()) {
         this->flushPendingCommands(kNormal_PlaybackMode);
     } else {
-        bool mustNotifyDirectly = !fCanDiscardCanvasContents;
         this->aboutToDraw();
-        if (mustNotifyDirectly) {
-            fSurface->notifyContentWillChange(SkSurface::kRetain_ContentChangeMode);
-        }
     }
 
     fImmediateCanvas->flush();