Add GpuCommandBuffer support.

src/gpu/GrGpuCommandBuffer.h

+/*
+* Copyright 2016 Google Inc.
+*
+* Use of this source code is governed by a BSD-style license that can be
+* found in the LICENSE file.
+*/
+
+#ifndef GrGpuCommandBuffer_DEFINED
+#define GrGpuCommandBuffer_DEFINED
+
+#include "GrColor.h"
+
+class GrRenderTarget;
+
+class GrGpuCommandBuffer {
+public:
+    enum LoadAndStoreOp {
+        kLoadAndStore_LoadAndStoreOp,
+        kLoadAndDiscard_LoadAndStoreOp,
+        kClearAndStore_LoadAndStoreOp,
+        kClearAndDiscard_LoadAndStoreOp,
+        kDiscardAndStore_LoadAndStoreOp,
+        kDiscardAndDiscard_LoadAndStoreOp,
+    };
+
+    GrGpuCommandBuffer() {}
+    virtual ~GrGpuCommandBuffer() {}
+
+    // Signals the end of recording to the command buffer and that it can now be submitted.
+    virtual void end() = 0;
+
+    // Sends the command buffer off to the GPU object to execute the commands built up in the
+    // buffer. The gpu object is allowed to defer execution of the commands until it is flushed.
+    virtual void submit() = 0;
+};
+
+#endif