Add discard support to Vulkan

src/gpu/vk/GrVkGpuCommandBuffer.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 GrVkGpuCommandBuffer_DEFINED
 #define GrVkGpuCommandBuffer_DEFINED
 
@@ skipping 14 matching lines @@
 public:
     GrVkGpuCommandBuffer(GrVkGpu* gpu,
                          GrVkRenderTarget*,
                          const LoadAndStoreInfo& colorInfo,
                          const LoadAndStoreInfo& stencilInfo);
 
     virtual ~GrVkGpuCommandBuffer();
 
     void end() override;
 
-    void discard(GrRenderTarget* rt) override {}
+    void discard(GrRenderTarget* rt) override;
 
 private:
     GrGpu* gpu() override;
 
     void onSubmit(const SkIRect& bounds) override;
 
     // Bind vertex and index buffers
     void bindGeometry(const GrPrimitiveProcessor&, const GrNonInstancedMesh&);
 
     sk_sp<GrVkPipelineState> prepareDrawState(const GrPipeline&,
@@ skipping 17 matching lines @@
     VkClearValue                fColorClearValue;
 
     SkTArray<GrVkImage*>        fSampledImages;
 
     bool                        fIsEmpty;
 
     typedef GrGpuCommandBuffer INHERITED;
 };
 
 #endif