Implement SkBaseDevice snapshot support

src/image/SkSurface_Gpu.h

Index: src/image/SkSurface_Gpu.h
diff --git a/src/image/SkSurface_Gpu.h b/src/image/SkSurface_Gpu.h
index 90c00c4b4570b0aacb2fbcdda07cbcd8981d9be4..0348d44235df57c7070425cb451000ba6229bc32 100644
--- a/src/image/SkSurface_Gpu.h
+++ b/src/image/SkSurface_Gpu.h
@@ -24,15 +24,20 @@ public:
     SkCanvas* onNewCanvas() SK_OVERRIDE;
     SkSurface* onNewSurface(const SkImageInfo&) SK_OVERRIDE;
     SkImage* onNewImageSnapshot(Budgeted) SK_OVERRIDE;
-    virtual void onDraw(SkCanvas*, SkScalar x, SkScalar y,
-                        const SkPaint*) SK_OVERRIDE;
-    void onCopyOnWrite(ContentChangeMode) SK_OVERRIDE;
-    void onDiscard() SK_OVERRIDE;
+    uint32_t generationID() SK_OVERRIDE;
 
     SkGpuDevice* getDevice() { return fDevice; }
 
+    void notifyContentWillChange() {
+        fGenerationID = 0;
+    }
+
+    // For the tests. Going through getDevice()->accessRenderTarget() will detach the snapshot,
+    // which most of the tests try to test.
+    bool isBackendBudgeted() const;
 private:
     SkGpuDevice* fDevice;
+    uint32_t fGenerationID;
 
     typedef SkSurface_Base INHERITED;
 };