This will allow each backend to implement this method separately. The
current set of ContextImpl methods used for robustness are slightly
D3D-centric.

BUG=angleproject:1463

Change-Id: I101f8ada2c49de4cf110db48b1e8380c52b50fb2
Reviewed-on: https://chromium-review.googlesource.com/365829Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>