Adds a theme promo in the lower right corner

BUG=None

TEST=The theme promo should show up in the lower right corner and when clicked should navigate to the Chrome theme gallery. The promo should go away after 5 runs of chrome
Review URL: http://codereview.chromium.org/217020

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@26894 0039d316-1c4b-4281-b951-d872f2087c98