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>