Add transfer buffer support.

src/gpu/GrGpu.h

Index: src/gpu/GrGpu.h
diff --git a/src/gpu/GrGpu.h b/src/gpu/GrGpu.h
index dd33cff39f166cef5a137208b0783f3ac9c87896..beeb01aced79d1caf4c7e66e72005b03998ca4f2 100644
--- a/src/gpu/GrGpu.h
+++ b/src/gpu/GrGpu.h
@@ -31,6 +31,7 @@ class GrRenderTarget;
 class GrStencilAttachment;
 class GrSurface;
 class GrTexture;
+class GrTransferBuffer;
 class GrVertexBuffer;
 class GrVertices;
 
@@ -128,6 +129,22 @@ public:
      */
     GrIndexBuffer* createIndexBuffer(size_t size, bool dynamic);
 
+    enum TransferType {
+        kCpuToGpu_TransferType,
+        kGpuToCpu_TransferType
+    };
+
+    /**
+     * Creates a transfer buffer.
+     *
+     * @param size      size in bytes of the index buffer
+     * @param toGpu     true if used to transfer from the cpu to the gpu
+     *                  otherwise to be used to transfer from the gpu to the cpu
+     *
+     * @return The transfer buffer if successful, otherwise nullptr.
+     */
+    GrTransferBuffer* createTransferBuffer(size_t size, TransferType type);
+    
     /**
      * Resolves MSAA.
      */
@@ -452,6 +469,7 @@ private:
                                                       GrWrapOwnership) = 0;
     virtual GrVertexBuffer* onCreateVertexBuffer(size_t size, bool dynamic) = 0;
     virtual GrIndexBuffer* onCreateIndexBuffer(size_t size, bool dynamic) = 0;
+    virtual GrTransferBuffer* onCreateTransferBuffer(size_t size, TransferType type) = 0;
 
     // overridden by backend-specific derived class to perform the clear.
     virtual void onClear(GrRenderTarget*, const SkIRect& rect, GrColor color) = 0;