Refactor gmock and gtest dependencies for ANGLE's tests. This change allows these targets to be referenced from Chromium's build projects. BUG=angleproject:833 Change-Id: I14694f610921d28ddcd8664887086ca30375b7a2 Reviewed-on: https://chromium-review.googlesource.com/231072 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Kenneth Russell <kbr@chromium.org> Tested-by: Kenneth Russell <kbr@chromium.org>