InspectorResourceAgent should hold a strong reference to replay XHRs

This CL is to unblock crrev.com/385963004

Before this patch, |InspectorResourceAgent::replayXHR| relied on
|XMLHTTPRequest::sendForInspectorXHRReplay| incrementing self reference
count via |ActiveDOMObject::setPendingActivity|.
This patch change InspectorResourceAgent to have a strong reference
against its replay XHRs, so that it would not depend on this behavior
any more.

BUG=353845

Review URL: https://codereview.chromium.org/387313008

git-svn-id: svn://svn.chromium.org/blink/trunk@178163 bbb929c8-8fbe-4397-9dbb-9b2b20218538