The overhead of the function call ended up being a hot spot for draw calls
since the attributes are iterated over many times in VertexDataManager.

BUG=angleproject:959

Change-Id: I6c11d4a5a22c314c69b3403dfdeb2d950b1c7639
Reviewed-on: https://chromium-review.googlesource.com/262336Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Nicolas Capens <capn@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>