GL: Temporarily disable parallel shader compile. The Skia bots have been unable to roll ANGLE since parallel compile landed in ANGLE. Speculatively disable parallel compile using a workaround. Failed roll: https://skia-review.googlesource.com/c/skia/+/184705 Also seen instability on Linux, Mac and Windows in 922936. BUG=922936 BUG=849576 Change-Id: I7116b3f6541425135984c52a8077bdb98d961784 Reviewed-on: https://chromium-review.googlesource.com/c/1415729 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>