Adreno's driver in Android Oreo follows the spec to the letter,
and applies "#pragma STDGL invariant(all)" only to the outputs of
a fragment shader, causing InvariantAllBoth test to fail.

Since the wording of the spec doesn't match the intended usage of this
pragma, ANGLE should flatten it to workaround spec conformant drivers.

BUG=angleproject:1293

Change-Id: I9f8ad44adc3b958204b88805dba8c5b7964b3d9f
Reviewed-on: https://chromium-review.googlesource.com/860920Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>