[Sync] Cleanup all tab sync enabling logic now that its on by default.

BUG=none
TEST=


Review URL: https://chromiumcodereview.appspot.com/10443046

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@139462 0039d316-1c4b-4281-b951-d872f2087c98
    ProfileSyncService::GetLastSessionSnapshot() const {
  if (backend_.get() && backend_initialized_) {
    return backend_->GetLastSessionSnapshot();
  }
  NOTREACHED();
  return browser_sync::sessions::SyncSessionSnapshot();
}

    ProfileSyncService::GetLastSessionSnapshot() const {
  if (backend_.get() && backend_initialized_) {
    return backend_->GetLastSessionSnapshot();
  }
  NOTREACHED();
  return browser_sync::sessions::SyncSessionSnapshot();
}
