GitLab

Currently, DevTools file watcher reports both added and changed paths
as "changed" and doesn't report removed paths.

This patch improves DevTools file watcher so that it starts reporting
changed, added and removed paths.

BUG=none
R=dgozman, pfeldman

Review-Url: https://codereview.chromium.org/2384343002
Cr-Commit-Position: refs/heads/master@{#422838}