Store channel bits info in angle::Format. This allows us to delete some duplicated code in the D3D11-side. BUG=angleproject:1389 BUG=angleproject:1459 Change-Id: Ifdcfcd4a56e06ff2ae8f5ca0bda72281d52c2964 Reviewed-on: https://chromium-review.googlesource.com/392208 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org>