Use FileDescriptorWatcher in AlarmTimer.

FileDescriptorWatcher is a new API that replaces
MessageLoopForIO::WatchFileDescriptor.

This CL also gets rid of the base::Thread created in the anonymous
namespace of alarm_timer_chromeos.cc. The FileDescriptorWatcher
API is supported by every TaskScheduler thread. Very soon, most
tasks in Chrome will run in TaskScheduler. Therefore, there is no
reason to create a base::Thread to allow AlarmTimer to be used from
threads that don't support the FileDescriptorWatcher API.

BUG=645114

Review-Url: https://codereview.chromium.org/2398753003
Cr-Original-Commit-Position: refs/heads/master@{#423617}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 8e17d7e9031b7e2e58473fe66a399943051357f0
3 files changed