Add wrapBackendTextureAsRenderTarget API

src/image/SkSurface_Gpu.h

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkSurface_Gpu_DEFINED
 #define SkSurface_Gpu_DEFINED
 
 #include "SkSurface_Base.h"
 
 #if SK_SUPPORT_GPU
 
 class SkGpuDevice;
 
 class SkSurface_Gpu : public SkSurface_Base {
 public:
     SkSurface_Gpu(SkGpuDevice*);
     virtual ~SkSurface_Gpu();
 
     GrBackendObject onGetTextureHandle(BackendHandleAccess) override;
     bool onGetRenderTargetHandle(GrBackendObject*, BackendHandleAccess) override;
     SkCanvas* onNewCanvas() override;
     SkSurface* onNewSurface(const SkImageInfo&) override;
     SkImage* onNewImageSnapshot(Budgeted) override;
     void onCopyOnWrite(ContentChangeMode) override;
     void onDiscard() override;
+    void onPrepareForExternalIO() override;
 
     SkGpuDevice* getDevice() { return fDevice; }
 
 private:
     SkGpuDevice* fDevice;
 
     typedef SkSurface_Base INHERITED;
 };
 
 #endif // SK_SUPPORT_GPU
 
 #endif // SkSurface_Gpu_DEFINED