-Add a new renderer and context type to own native EGL contexts and
 handle destruction.
-Track the current EGL surface and context per-thread.
-Support unvirtualized contexts by creating a new context for every client
 context.

BUG=angleproject:2464

Change-Id: Ib2efa1d88c771b4a78625e0e3546f6ed95678c91
Reviewed-on: https://chromium-review.googlesource.com/1110943
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>