Revert "Associate PingLoader loaders with routes in the browser."

Associating them with routes introduces the possibility of them getting lost in
the Chromium ResourceScheduler. (If the request was deferred in
pending_requests and never started before the route was removed.) Just revert
it for now since the histograms this was needed for will be done differently.

> Ping requests currently do not pass through willSendRequest which means they
> are not correlated with a RenderFrameHost in the browser. Change this so they
> can be.
>
> BUG=302816
>
> Review URL: https://codereview.chromium.org/171083003

BUG=302816

Review URL: https://codereview.chromium.org/178363008

git-svn-id: svn://svn.chromium.org/blink/trunk@167884 bbb929c8-8fbe-4397-9dbb-9b2b20218538