[Mac] Implement the update available notification in the wrench menu.

The changes to restart_browser.mm also affect the sheet displayed in the About
window. Note that the badge on the wrench menu looks bad.

BUG=45147
TEST=difficult

Review URL: http://codereview.chromium.org/2856042

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@52911 0039d316-1c4b-4281-b951-d872f2087c98
8 files changed