Implement app process model isolation.

      The process grouping logic is unfortunately duplicated in SiteInstance and
RenderView. URLs that are part of extension X's web extent get converted into
a pseudo URL of the form chrome-extension://X/path. This groups pages from an
extension app and its offline resources into the same process.

The rest is mostly plumbing and passing data around.

BUG=41273

Review URL: http://codereview.chromium.org/1735004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@45384 0039d316-1c4b-4281-b951-d872f2087c98