Prepare for using two backends in SyncFileSystemService, part 4

As a preparation for using multiple RemoteFileSyncService.

- Moved SyncFileSystemService::SyncRunner implementation into its own file, sync_process_runner.* and renamed to SyncProcessRunner
- Added LocalSyncRunner and RemoteSyncRunner class, both extend SyncProcessRunner
- Let LocalSyncRunner implement LocalFileSyncService::Observer
- Let RemoteSyncRunner implement RemoteFileSyncService::Observer

The plan is: we're going to have two RemoteFileSyncService and RemoteSyncRunner
for v1 and v2.

No functional changes yet.

BUG=324215
TEST=manual (tested some remote and local sync)

Review URL: https://codereview.chromium.org/95023003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237886 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237886