ESSL1 states that it is an error to have a non-preprocessor token
before an extension pragma but ANGLE would mark a non-preprocessor
token as seen even if it was in a disabled if-block.

Bug: b/161716126
Change-Id: I782224bee211bc7214a2d1d98d4829b4c94101bd
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2547205Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>