Fetch last change (revision) info in a separate action that can run

every build, instead of having it occur as a side effect of updating
some other target for which we want to use normal up-to-date checks.
BUG=none
TEST=none
Review URL: http://codereview.chromium.org/118192

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@17634 0039d316-1c4b-4281-b951-d872f2087c98