Prepare for using both backends in single SyncFileSystemService, part 2

- Changes (fixes) GetSyncFileSystemDir() behavior to return non-dev dir if V2 is globally enabled
- Adds '--syncfs-v2-whitelist' command line to allow specifying whitelisted app IDs at run-time, mainly for testing
- Moves SyncEngine creation code from SyncFileSystemServiceFactory to RemoteFileSyncService::CreateForBrowserContext(version)
- Changes the database name for v2 ("DriveMetadata" -> "DriveMetadata_v2") so that v1 and v2 db can coexist

Almost no functional changes yet, except for GetSyncFileSystemDir() behavior fix

BUG=324215
R=tzik

Review URL: https://codereview.chromium.org/94263002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237799 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237799