Add keepalive support to drivefs API

In some situations mounting drivefs may take very long time. To
distinguish it from a total hang we send periodic keepalives from drivefs.

BUG=chromium:899746

Change-Id: Iee906651557a8f8eab62d58298f33c7c3e61724e
Reviewed-on: https://chromium-review.googlesource.com/c/1305253
Commit-Queue: Sergei Datsenko <dats@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603732}
  void MaybeNotifyDelegateOnMounted() {
    if (mounted()) {
      host_->timer_->Stop();
      host_->mount_observer_->OnMounted(mount_path());
    }
  }

  void MaybeNotifyDelegateOnMounted() {
    if (mounted()) {
      host_->timer_->Stop();
      host_->mount_observer_->OnMounted(mount_path());
    }
  }
