Remove support for GL discards

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"
 
 #include "GrGLGpu.h"
 
 class GrGLGpuCommandBuffer : public GrGpuCommandBuffer {
 /**
  * We do not actually buffer up draws or do any work in the this class for GL. Instead commands
  * are immediately sent to the gpu to execute. Thus all the commands in this class are simply
  * pass through functions to corresponding calls in the GrGLGpu class.
  */
 public:
     GrGLGpuCommandBuffer(GrGLGpu* gpu) : fGpu(gpu) {}
 
     virtual ~GrGLGpuCommandBuffer() {}
 
     void end() override {}
 
-    void discard(GrRenderTarget* rt) override { fGpu->discard(rt); }
+    void discard(GrRenderTarget* rt) override {}
 
 private:
     GrGpu* gpu() override { return fGpu; }
 
     void onSubmit(const SkIRect& bounds) override {}
 
     void onDraw(const GrPipeline& pipeline,
                 const GrPrimitiveProcessor& primProc,
                 const GrMesh* mesh,
                 int meshCount) override {
         fGpu->draw(pipeline, primProc, mesh, meshCount);
     }
 
     void onClear(GrRenderTarget* rt, const SkIRect& rect, GrColor color) override {
         fGpu->clear(rect, color, rt);
     }
 
     void onClearStencilClip(GrRenderTarget* rt, const SkIRect& rect, bool insideClip) override {
         fGpu->clearStencilClip(rect, insideClip, rt);
     }
 
     GrGLGpu*                    fGpu;
 
     typedef GrGpuCommandBuffer INHERITED;
 };
 
 #endif