To support both virtualized and unvirtualized contexts, the RendererGL object
represents the native context.  Update ContexGL and the various DisplayGLs to
hold RendererGL  with a shared_ptr so that deletion of the renderer happens at
the correct time.

Update RendererGL to take ownership of FunctionsGL.

BUG=angleproject:2464

Change-Id: Id040a8053973d73936c0a7ff0ab5edb1a3f16dc6
Reviewed-on: https://chromium-review.googlesource.com/1085851Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>