Use googletest flag macros to access googletest flags. The implementation details of flags can change; fixing this proactively makes it easier to roll googletest. Bug: chromium:1409870 Change-Id: I5efa2487d9e38e122ca20f201748faf8e8050347 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5092928 Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Auto-Submit: Daniel Cheng <dcheng@chromium.org>