Revert of remove workaround for dx9 angle slow rgba pixel ops

src/gpu/GrGpu.h

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrGpu_DEFINED
 #define GrGpu_DEFINED
 
@@ skipping 130 matching lines @@
     virtual GrPixelConfig preferredReadPixelsConfig(GrPixelConfig readConfig,
                                                     GrPixelConfig surfaceConfig) const {
         return readConfig;
     }
     virtual GrPixelConfig preferredWritePixelsConfig(GrPixelConfig writeConfig,
                                                      GrPixelConfig surfaceConfig) const {
         return writeConfig;
     }
 
     /**
-     * Called before uploading writing pixels to a GrTexture.
+     * Called before uploading writing pixels to a GrTexture when the src pixel config doesn't
+     * match the texture's config.
      */
     virtual bool canWriteTexturePixels(const GrTexture*, GrPixelConfig srcConfig) const = 0;
 
     /**
      * OpenGL's readPixels returns the result bottom-to-top while the skia
      * API is top-to-bottom. Thus we have to do a y-axis flip. The obvious
      * solution is to have the subclass do the flip using either the CPU or GPU.
      * However, the caller (GrContext) may have transformations to apply and can
      * simply fold in the y-flip for free. On the other hand, the subclass may
      * be able to do it for free itself. For example, the subclass may have to
@@ skipping 304 matching lines @@
     GrTraceMarkerSet                                                    fActiveTraceMarkers;
     GrTraceMarkerSet                                                    fStoredTraceMarkers;
     // The context owns us, not vice-versa, so this ptr is not ref'ed by Gpu.
     GrContext*                                                          fContext;
 
     friend class GrPathRendering;
     typedef SkRefCnt INHERITED;
 };
 
 #endif