This allows us to pass some dEQP GLES 2.0 tests that require using an ES 3.0
context and matches native driver behaviour of returning a higher-than-
requested context version when it's backwards compatible.

BUG=angleproject:3425
BUG=angleproject:3300
BUG=angleproject:3715
BUG=angleproject:3243
BUG=angleproject:3240
BUG=angleproject:3716

Change-Id: I03dfd56deb6191c3363808717e0eea6fdcf5f3ac
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1709111
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>