Revert 232802 "[Net] Assert that URLRequests with LOAD_IGNORE_LI..."

> [Net] Assert that URLRequests with LOAD_IGNORE_LIMITS have MAXIMUM_PRIORITY
> 
> Also remove the priority juggling in ClientSocketPoolBaseHelper, and instead
> assert that any requests with ignore_limits = true also has priority =
> MAXIMUM_PRIORITY.
> 
> BUG=166689
> R=asanka@chromium.org, gene@chromium.org, joaodasilva@chromium.org, mmenke@chromium.org, simonjam@chromium.org
> TBR=benm@chromium.org, jamesr@chromium.org
> 
> Review URL: https://codereview.chromium.org/51683002

Broke PrerenderBrowserTest.PrerenderDeferredSynchronousXHR. Running locally I was hitting a check on url_request.cc:979.

TBR=akalin@chromium.org

Review URL: https://codereview.chromium.org/47563006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232832 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232832