remove premature discard optimization

src/image/SkSurface_Gpu.cpp

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkSurface_Gpu.h"
 
 #include "SkCanvas.h"
@@ skipping 15 matching lines @@
     fDevice->unref();
 }
 
 GrBackendObject SkSurface_Gpu::onGetTextureHandle(TextureHandleAccess access) {
     GrRenderTarget* rt = fDevice->accessRenderTarget();
     switch (access) {
         case kFlushRead_TextureHandleAccess:
             rt->prepareForExternalRead();   // todo: rename to prepareForExternalAccess()
             break;
         case kFlushWrite_TextureHandleAccess:
+        case kDiscardWrite_TextureHandleAccess:
+            // for now we don't special-case on Discard, but we may in the future.
             this->notifyContentWillChange(kRetain_ContentChangeMode);
             rt->flushWrites();
             break;
-        case kDiscardWrite_TextureHandleAccess:
-            this->notifyContentWillChange(kDiscard_ContentChangeMode);
-            rt->discard();
-            break;
     }
     return rt->asTexture()->getTextureHandle();
 }
 
 SkCanvas* SkSurface_Gpu::onNewCanvas() {
     SkCanvas::InitFlags flags = SkCanvas::kDefault_InitFlags;
     // When we think this works...
 //    flags |= SkCanvas::kConservativeRasterClip_InitFlag;
 
     return SkNEW_ARGS(SkCanvas, (fDevice, flags));
@@ skipping 81 matching lines @@
     }
     SkAutoTUnref<SkGpuDevice> device(SkGpuDevice::Create(surface->asRenderTarget(), props,
                                                          SkGpuDevice::kUninit_InitContents));
     if (!device) {
         return NULL;
     }
     return SkNEW_ARGS(SkSurface_Gpu, (device));
 }
 
 #endif