[SyncFS] Handle remote sync-root deletion

On sync-root deletion, SyncEngine drops all entries of MetadataDatabase and tries to create sync root again.

BUG=240165
TEST=manual
NOTRY=true

Review URL: https://codereview.chromium.org/112563002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240066 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240066