We now have the GLHelperReadBackSupport, this should tell us

if some format is supported or not.IsReadbackConfigSupported
takes care of validating this.

BUG=376769, 415131

Review URL: https://codereview.chromium.org/570003002

Cr-Commit-Position: refs/heads/master@{#295656}

content/browser/compositor/delegated_frame_host.cc

@@ -574,6 +574,9 @@ void DelegatedFrameHost::PrepareTextureCopyOutputResult(
   base::ScopedClosureRunner scoped_callback_runner(
       base::Bind(callback, false, SkBitmap()));
 
+  // TODO(sikugu): We should be able to validate the format here using
+  // GLHelper::IsReadbackConfigSupported before we processs the result.
+  // See crbug.com/415682.
   scoped_ptr<SkBitmap> bitmap(new SkBitmap);
   if (!bitmap->tryAllocPixels(SkImageInfo::Make(dst_size_in_pixel.width(),
                                                 dst_size_in_pixel.height(),

content/common/gpu/client/gl_helper_readback_support.cc

@@ -25,6 +25,8 @@ void GLHelperReadbackSupport::InitializeReadbackSupport() {
   for (int i = 0; i <= kLastEnum_SkColorType; ++i) {
     format_support_table_[i] = GLHelperReadbackSupport::NOT_SUPPORTED;
   }
+  // TODO(sikugu): kAlpha_8_SkColorType support check is failing on mesa.
+  // See crbug.com/415667.
   CheckForReadbackSupport(kRGB_565_SkColorType);
   CheckForReadbackSupport(kARGB_4444_SkColorType);
   CheckForReadbackSupport(kRGBA_8888_SkColorType);