Moved all state from SiteIsolationPolicy out to the PendingRequestInfo struct in the ResourceScheduler

This removes a few redundant map lookups, simplifies the logic, and also paves the way for using this class from multiple threads.
R=creis@chromium.org
BUG=277124

Review URL: https://codereview.chromium.org/187063006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255389 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255389