Swap render target instead of creating a new gpu device for surface copy-on-write

src/gpu/SkGpuDevice.h

Index: src/gpu/SkGpuDevice.h
diff --git a/src/gpu/SkGpuDevice.h b/src/gpu/SkGpuDevice.h
index 4830c026ac1a41caee7ff2ff36fec0649a19a4f7..cd2dcdc71c4a830e7135b9797eb4378b75100a02 100644
--- a/src/gpu/SkGpuDevice.h
+++ b/src/gpu/SkGpuDevice.h
@@ -62,6 +62,8 @@ public:
     // set all pixels to 0
     void clearAll();
 
+    void replaceRenderTarget(bool shouldRetainContent);
+
     GrRenderTarget* accessRenderTarget() SK_OVERRIDE;
 
     SkImageInfo imageInfo() const SK_OVERRIDE {
@@ -199,6 +201,9 @@ private:
 
     static SkPicture::AccelData::Key ComputeAccelDataKey();
 
+    static GrRenderTarget* CreateRenderTarget(GrContext*, SkSurface::Budgeted, const SkImageInfo&,
+                                              int sampleCount);
+
     typedef SkBaseDevice INHERITED;
 };