This change allows extension directive after ESSL1.0 non-preprocessor tokens on CHROMEOS.
CHROMEOS kiosks have a video player that violates this spec requirement so just flagging
as a warning to prevent failed video playback.

Added ANGLE_PLATFORM_CHROMEOS define to build config in order to only make this a warning
on CHROMEOS. Split the ESSL1 & ESSL3 cases back out as ESSL3 had always been an error
on all platforms in the past and so want to keep it that way.

Bug: 1003005
Bug: angleproject:4023
Change-Id: Ia931b3a8dad82dbda4c9c9e49a9c1090116397b5
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1866464Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Tobin Ehlis <tobine@google.com>
(cherry picked from commit 93121f36)
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1875147Reviewed-by: Tobin Ehlis <tobine@google.com>