Add comment about leaked FileDescriptorWatcher::Controller::Watcher.

If the MessageLoopForIO is deleted before Watcher::StartWatching() runs,
|watcher_| is leaked. If the MessageLoopForIO is deleted after
Watcher::StartWatching() runs but before the DeleteSoon task runs,
|watcher_| is deleted from Watcher::WillDestroyCurrentMessageLoop().

BUG=645114

Review-Url: https://codereview.chromium.org/2382443011
Cr-Commit-Position: refs/heads/master@{#422320}
1 file changed