[SyncFS] Replace LOG_INFO to LOG_VERBOSE in SyncFS stuff

BUG=331988

Review URL: https://codereview.chromium.org/150043002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247931 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247931
drive: Also lock SearchOperation when locking ChangeListLoader

Split LoaderController from ChangeListLoader.
SearchOperation obeys LoaderController.

BUG=260539
TEST=unit_tests

Review URL: https://codereview.chromium.org/150113002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247932 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247932