Allow client to force an SkImage snapshot to be unique (and uniquely own its backing store).

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 23241540bfe3d95d7f550480204d230f77f14f06..23aed2cd3c5d020688b70724d82c7919df1619c8 100644
--- a/src/image/SkSurface_Gpu.h
+++ b/src/image/SkSurface_Gpu.h
@@ -23,7 +23,7 @@ public:
     bool onGetRenderTargetHandle(GrBackendObject*, BackendHandleAccess) override;
     SkCanvas* onNewCanvas() override;
     SkSurface* onNewSurface(const SkImageInfo&) override;
-    SkImage* onNewImageSnapshot(Budgeted) override;
+    SkImage* onNewImageSnapshot(Budgeted, ForceCopyMode) override;
     void onCopyOnWrite(ContentChangeMode) override;
     void onDiscard() override;