[SyncFS] Refactor MetadataDatabase on dirty flag handling and sync-root creation

 * Make leveldb::WriteBatch nullable to reduce extra write.
 * Consolidate dirty flag handling.
 * Simplify sync-root creation

BUG=331988
NOTRY=true

Review URL: https://codereview.chromium.org/134863003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248974 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248974