Place RefPtr for protection not in internalAbort() but in its caller.

We have RefPtr for preventing XHR from being destroyed in
internalAbort(). But we're touching XHR's member variables in the caller
of internalAbort(). We must have the RefPtr for protection there.

BUG=421504

Review URL: https://codereview.chromium.org/663693003

git-svn-id: svn://svn.chromium.org/blink/trunk@183859 bbb929c8-8fbe-4397-9dbb-9b2b20218538