This extension allows the user to specify if ANGLE should create a
context of the exact requested version or a higher version context that
is backwards compatible.

BUG=angleproject:3425

Change-Id: I7d0b75cdd7e34a2fc888aa238e1eeb67af82ae0d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1601560
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>