Display WGL is the best place to track the currently bound surfaces and
contexts and cache them appropriately.  This results in a ~40% speed
increase in the WebGL Aqarium demo because wglMakeCurrent is now only
called once at initialization.

BUG=angleproject:1651

Change-Id: I61132bb4a0db8de592600173c20787c67d18e067
Reviewed-on: https://chromium-review.googlesource.com/543583
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>