[SyncFS] Add TaskRunner to SyncEngineContext

 * Inject TaskRunner used in RemoteToLocalSyncer and LocalToRemoteSyncer.
 * Use SequencedWorkerPool instead of FILE thread.

BUG=240165
R=nhiroki@chromium.org

Review URL: https://codereview.chromium.org/76153003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235961 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235961