This fixes the build of angle_end2end_tests on Linux/Clang.

BUG=angleproject:997

Change-Id: Ie6eca3dc0fc7114d2755892b4b02e8b47a8fa478
Reviewed-on: https://chromium-review.googlesource.com/270680Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>