Fixes the message of search hint in app-list.

BUG=347204
TBR=kuscher@chromium.org, xiyuan@chromium.org
TEST=None

Review URL: https://codereview.chromium.org/181723004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253694 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253694