ANGLE devs can keep tabs on subfolders like in Chromium.

Bug: none
Change-Id: Ib507b758d2ee3b00808e798c23361f56930b3f95
Reviewed-on: https://chromium-review.googlesource.com/c/1495034Reviewed-by: Jamie Madill <jmadill@google.com>
Commit-Queue: Jamie Madill <jmadill@google.com>