Update clear calls to return Error objects instead of calling gl::error. BUG=angle:520 Change-Id: I474a6ed29b882963f7f3425515e7d65f8f69b3e4 Reviewed-on: https://chromium-review.googlesource.com/211440 Reviewed-by: Shannon Woods <shannonwoods@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>