Add support to ProxyService for downloading a PAC script on behalf of the ProxyResolver. A ProxyResolver can select this new behavior by subclassing ProxyResolver with |does_fetch = false|. A consequence of this change is that proxy resolve requests are maintained in a queue by ProxyService (rather than implicitly in a queue on the PAC thread's message loop). This simplifies cancellation.This solves issue 7461, and is work-in-progress towards {2764, 74}BUG=7461

      Review URL: http://codereview.chromium.org/21328

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@10197 0039d316-1c4b-4281-b951-d872f2087c98