Adds a RendererWGL which owns the native WGL GL context.

Adds a ContextWGL which owns the RendererWGL and communicates the native WGL
context back to DisplayWGL for making current.

BUG=angleproject:2464

Change-Id: Ideb11c164da2c304f860c544a56450d0ad8fafac
Reviewed-on: https://chromium-review.googlesource.com/1038131
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>