Passing the Renderer pointer instead of the Context pointer makes
these methods a bit easier to work with from the "EGL" sections of
the code. This is a refactoring-only change to aid the Depth/Stencil
implementation.

Bug: angleproject:2357
Change-Id: Icbcc72a1daff4edd947a21672744498781cfc064
Reviewed-on: https://chromium-review.googlesource.com/919523Reviewed-by: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>