GitLab

- Suppresses several -Wunreachable-code warnings
- Makes it clearer this is a build-time constant
- Removes half a use of GOOGLE_CHROME_BUILD in a .h file

No intended behavior change.

Bug: 346399,961769
Change-Id: Idb574bc2bb8e55e9b060f7fd743242904b291725
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2120269
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753612}