Implements ES 3.1 support.

Bug: angleproject:4029
Change-Id: I3c08602cf4ec64621bfdb40cd40d142b666e6edf
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2046052
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>