[SyncFS] Force activate folder tracker on folder creation conflict case

If a folder activation failed due to existing item, the folder is eventually deleted.
This can be happen in some edge case.

This CL should fix it.

BUG=331988
NOTRY=true

Review URL: https://codereview.chromium.org/148483009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250049 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250049