This allows caching the current surface more efficiently than at the
egl layer because we can take advantage of having only one backing
context.

BUG=angleproject:1651

Change-Id: I62867b16ac5e06901a988dc41a3d4812accdb74c
Reviewed-on: https://chromium-review.googlesource.com/543835
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>