Add GpuCommandBuffer support.

src/gpu/gl/GrGLGpuCommandBuffer.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 GrGLGpuCommandBuffer_DEFINED
+#define GrGLGpuCommandBuffer_DEFINED
+
+#include "GrGpuCommandBuffer.h"
+
+class GrGLGpu;
+
+class GrGLGpuCommandBuffer : public GrGpuCommandBuffer {
+public:
+    GrGLGpuCommandBuffer(GrGLGpu* gpu) /*: fGpu(gpu)*/ {}
+
+    virtual ~GrGLGpuCommandBuffer() {}
+
+    void end() override {}
+
+    void submit() override {}
+
+private:
+    // commented out to appease clang compiler warning about unused private field
+   // GrGLGpu*                    fGpu;
+
+    typedef GrGpuCommandBuffer INHERITED;
+};
+
+#endif
+