Maintain minimal error response.

content/browser/compositor/delegated_frame_host.cc

Index: content/browser/compositor/delegated_frame_host.cc
diff --git a/content/browser/compositor/delegated_frame_host.cc b/content/browser/compositor/delegated_frame_host.cc
index 2aa746dfc9e2a0ccc597e4b6e48b1ba2ee941746..20d896f37d1e8e08771982c1129989ead5604c31 100644
--- a/content/browser/compositor/delegated_frame_host.cc
+++ b/content/browser/compositor/delegated_frame_host.cc
@@ -586,11 +586,13 @@ void DelegatedFrameHost::PrepareTextureCopyOutputResult(
   // GLHelper::IsReadbackConfigSupported before we processs the result.

[no sievers, 2015/05/06 23:08:03]

What keeps us from doing this here? And falling back to RGBA if not supported?
Glancing over the bug the problem seemed to arise if we did this earlier before
the compositor is initialized, but here we should be ok, right?

I'm just wondering... this can be done in a separate patch, esp. if it might
still cause some problem we'd want to isolate.

[sivag, 2015/05/07 14:14:20]

Actually this fails in the browser tests, where some snapshot is requested
before the compositor gets initialized, may be we can check this separately.

ptal at the below issue raised for this
http://code.google.com/p/chromium/issues/detail?id=415131

   // 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(),
-                                                color_type,
-                                                kOpaque_SkAlphaType)))
+  if (!bitmap->tryAllocPixels(SkImageInfo::Make(
+          dst_size_in_pixel.width(), dst_size_in_pixel.height(), color_type,
+          kOpaque_SkAlphaType))) {
+    scoped_callback_runner.Reset(base::Bind(
+        callback, SkBitmap(), content::READBACK_BITMAP_ALLOCATION_FAILURE));
     return;
+  }
 
   ImageTransportFactory* factory = ImageTransportFactory::GetInstance();
   GLHelper* gl_helper = factory->GetGLHelper();
@@ -627,13 +629,13 @@ void DelegatedFrameHost::PrepareTextureCopyOutputResult(
 // static
 void DelegatedFrameHost::PrepareBitmapCopyOutputResult(
     const gfx::Size& dst_size_in_pixel,
-    const SkColorType color_type,
+    const SkColorType request_color_type,
     ReadbackRequestCallback& callback,
     scoped_ptr<cc::CopyOutputResult> result) {
+  SkColorType color_type = request_color_type;
   if (color_type != kN32_SkColorType && color_type != kAlpha_8_SkColorType) {
-    NOTIMPLEMENTED();
-    callback.Run(SkBitmap(), READBACK_FORMAT_NOT_SUPPORTED);
-    return;
+    // Switch back to default colortype if format not supported.
+    color_type = kN32_SkColorType;
   }
   DCHECK(result->HasBitmap());
   scoped_ptr<SkBitmap> source = result->TakeBitmap();
@@ -663,7 +665,7 @@ void DelegatedFrameHost::PrepareBitmapCopyOutputResult(
   bool success = grayscale_bitmap.tryAllocPixels(
       SkImageInfo::MakeA8(scaled_bitmap.width(), scaled_bitmap.height()));
   if (!success) {
-    callback.Run(SkBitmap(), content::READBACK_MEMORY_ALLOCATION_FAILURE);
+    callback.Run(SkBitmap(), content::READBACK_BITMAP_ALLOCATION_FAILURE);
     return;
   }
   SkCanvas canvas(grayscale_bitmap);