GitLab

In GYP we used to add all sources and then remove the unneeded ones.
Switch to addition which requires only listing the sources in one place.

Bug: 1026406

Change-Id: Ib085e35b59d3141e85b3265a0e849092adb9d1d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1943369Reviewed-by: Kevin Schoedel <kpschoedel@chromium.org>
Commit-Queue: Michael Spang <spang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720294}