Shutdown button tooltip string  changed for consistency.

BUG=331712
NOTRY=true

Review URL: https://codereview.chromium.org/147023003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247678 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247678