Show Pin option in app launcher context menu only when Shelf is present

Pin to Shelf option should only be present when applicable, i.e., in ChromeOS
and Windows in Metro Mode with the Ash shelf.

BUG=303571
TEST=Ensure Pin option is not visible in Windows 7 or Windows 8, except in
     Windows 8 multi-window Metro mode. Option should still be visible in
     ChromeOS.
R=Mr4D@chromium.org, xiyuan@chromium.org, tapted@chromium.org

Review URL: https://codereview.chromium.org/25929002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227070 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227070