Associate PingLoader loaders with routes in the browser.

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

git-svn-id: svn://svn.chromium.org/blink/trunk@167528 bbb929c8-8fbe-4397-9dbb-9b2b20218538