This will allow us to pull out the Renderer from the Context in more
places in Buffer11, for state update. Impacts a few method calls in
a few places.

BUG=angleproject:2151

Change-Id: I1360caea65a94d3de4cd9f52d1b74b10439b02b3
Reviewed-on: https://chromium-review.googlesource.com/673136Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>