GitLab

Adds a new "constants" source_set to avoid any chrome/app
dependencies.

Bug: 987646
Change-Id: I8fc61687cabc0618e43a7dcc92c30c5cf694e6af
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1906893Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715020}