TEST=webgl_conformance_gl_passthrough_tests on Linux/Windows

BUG=angleproject:2231

Change-Id: I17c7d693e2b75726e2478925e0f22963de6ab819
Reviewed-on: https://chromium-review.googlesource.com/763987Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>