EGL: Support unvirtualized contexts and unsafe multithreading. -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>