Handles the gl::Context class and its implementation.

Bug: angleproject:2491
Change-Id: I7b7eb0897d131a194f6b563b7e01756fd7adb7e1
Reviewed-on: https://chromium-review.googlesource.com/c/1280742Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>