Show menu after selection change

Delay showing the context menu shown from the context menu button. This
is so that the selection changes before the menu is shown and the update
of the menu items is done in the correct order.

BUG=301584
R=dbeam@chromium.org

Review URL: https://codereview.chromium.org/25174005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226217 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226217