This removes a lot of repeated code in tcuANGLEPlatform.cpp.

Bug: angleproject:2552
Change-Id: Ie4cc53af2dfdf719b78d272cbd260cab5b2efd7b
Reviewed-on: https://chromium-review.googlesource.com/1064196
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>