Sync FileSystem: Add DriveFileSyncClientInterface.

Add the interface class for the DriveFileSyncClient to test DriveFileSyncService.

BUG=176904
TBR=thestig@chromium.org


Review URL: https://chromiumcodereview.appspot.com/12255061

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@183212 0039d316-1c4b-4281-b951-d872f2087c98
7 files changed