Fix WritePixels test on ANGLE.

Don't upload BGRA to RGBA if not supported (ES2 w/ EXT BGRA extension).

R=senorblanco@chromium.org
Review URL: https://codereview.appspot.com/7305046

git-svn-id: http://skia.googlecode.com/svn/trunk@7622 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/include/gpu/GrContext.h b/include/gpu/GrContext.h
index dc69766..3b5e9c4 100644
--- a/include/gpu/GrContext.h
+++ b/include/gpu/GrContext.h
@@ -531,8 +531,11 @@
      * @param rowBytes      number of bytes between consecutive rows. Zero means rows are tightly
      *                      packed.
      * @param pixelOpsFlags see PixelOpsFlags enum above.
+     *
+     * @return true if the write succeeded, false if not. The write can fail because of an
+     *         unsupported combination of target and pixel configs.
      */
-    void writeRenderTargetPixels(GrRenderTarget* target,
+    bool writeRenderTargetPixels(GrRenderTarget* target,
                                  int left, int top, int width, int height,
                                  GrPixelConfig config, const void* buffer,
                                  size_t rowBytes = 0,
@@ -572,8 +575,10 @@
      * @param rowBytes      number of bytes between consecutive rows. Zero
      *                      means rows are tightly packed.
      * @param pixelOpsFlags see PixelOpsFlags enum above.
+     * @return true if the write succeeded, false if not. The write can fail because of an
+     *         unsupported combination of texture and pixel configs.
      */
-    void writeTexturePixels(GrTexture* texture,
+    bool writeTexturePixels(GrTexture* texture,
                             int left, int top, int width, int height,
                             GrPixelConfig config, const void* buffer,
                             size_t rowBytes,