Add ANGLE backend flag to about:flags

The flag is only available on Windows and offers choices to use the
OpenGL, D3D9 or D3D11 backend.

TEST=manual test on Windows
BUG=chromium:693090

Change-Id: Ia9ce5d58a0658af513b070e564014398f35a243e
Reviewed-on: https://chromium-review.googlesource.com/1202203
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589480}
4 files changed