Add a comment to generic_stop.icon to explain why it's not stop.icon

The file was added as stop.icon which broke some builds, was
renamed generic_stop.icon to unbreak those builds.
See https://chromium-review.googlesource.com/c/chromium/src/+/1261042

R=estade@chromium.org

Bug: 877702
Change-Id: If90932dac4868053748e25f2d0f0dd15876087cc
Reviewed-on: https://chromium-review.googlesource.com/c/1269730
Reviewed-by: Evan Stade <estade@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#597928}
1 file changed