fix slowness of getImageData() for CanvasRenderingContext2D in linux due to un-optimized format for…

src/gpu/GrGpu.h

Index: src/gpu/GrGpu.h
diff --git a/src/gpu/GrGpu.h b/src/gpu/GrGpu.h
index ade64312b866ac0f831ad9b20e413143cd6c1ba9..467462a42b4f78bfff36c866ec9844dd24b0b674 100644
--- a/src/gpu/GrGpu.h
+++ b/src/gpu/GrGpu.h
@@ -153,10 +153,10 @@ public:
 
     /**
      * Gets a preferred 8888 config to use for writing / reading pixel data. The returned config
-     * must have at least as many bits per channel as the config param.
+     * must have at least as many bits per channel as the readConfig or writeConfig param.
      */
-    virtual GrPixelConfig preferredReadPixelsConfig(GrPixelConfig config) const { return config; }
-    virtual GrPixelConfig preferredWritePixelsConfig(GrPixelConfig config) const { return config; }
+    virtual GrPixelConfig preferredReadPixelsConfig(GrPixelConfig readConfig, GrPixelConfig targetConfig) const { return readConfig; }
+    virtual GrPixelConfig preferredWritePixelsConfig(GrPixelConfig writeConfig, GrPixelConfig targetConfig) const { return writeConfig; }
 
     /**
      * Called before uploading writing pixels to a GrTexture when the src pixel config doesn't