XMLHTTPRequest should not use deprecated {,un}setPendingActivity

This CL also removes old lifetime hacks which were needed in old design.

DropProtectionAsync hack is removed. This was introduced to avoid "delete this" inside |ActiveDOMObject::stop()|, but from r178163, an external strong ref to "this" is guaranteed from V8 wrapper or the InspectorResourceAgent, so the hack is no longer needed.

Likewise, internalAbort() is guaranteed to not "delete this" anymore. It will make |hasPendingActivity() == false|, but the reference to the XHR will be still held by an V8 wrapper or InspectorResourceAgent. The destruction of XHR will be done at their GC timing.

BUG=354845

Review URL: https://codereview.chromium.org/385963004

git-svn-id: svn://svn.chromium.org/blink/trunk@178253 bbb929c8-8fbe-4397-9dbb-9b2b20218538