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..5b556c5b2543607e40e042263d3ba7d54d6bf458 100644
--- a/src/gpu/GrGpu.h
+++ b/src/gpu/GrGpu.h
@@ -152,11 +152,18 @@ public:
     void forceRenderTargetFlush();
 
     /**
-     * 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.
+     * Gets a preferred 8888 config to use for writing/reading pixel data to/from a surface with
+     * config surfaceConfig. The returned config 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 surfaceConfig) const {
+        return readConfig;
+    }
+    virtual GrPixelConfig preferredWritePixelsConfig(GrPixelConfig writeConfig,
+                                                     GrPixelConfig surfaceConfig) const {
+        return writeConfig;
+    }
 
     /**
      * Called before uploading writing pixels to a GrTexture when the src pixel config doesn't