Add a user action metric for showing the app menu with an accelerator.

BUG=321695

Review URL: https://codereview.chromium.org/104493004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238988 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238988