Update gl2ext.h. Also split the ANGLE modifications to the header into a separate include. BUG=angleproject:1897 Change-Id: I06cdcc26bab6bfa4b5bcb488c46558583cab5a83 Reviewed-on: https://chromium-review.googlesource.com/560020 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>