Files.app: Hanldes special search in setup for default path.

Previously, Files.app does not handle special search in setup for default path.
This CL add a condition check for special search.  Along the way, this CL
introduces AsyncUtil.Queue to reduce the number of indent.

BUG=326879
TEST=Follow the instruction of the bug, setting default path manually
for testing three code paths.

Review URL: https://codereview.chromium.org/109923002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240068 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240068