AppCache URLFetcher refactoring. Just pulling the URLRequest handling

      logic out of class AppCacheUpdateJob and into a seperate URLFetcher class.
There should be no functional changes here, just refactoring for the sake
of maintainability to (hopefully) make future work easier.

BUG=none
TEST=existing tests apply

Review URL: http://codereview.chromium.org/6486010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@74998 0039d316-1c4b-4281-b951-d872f2087c98