InspectorResourceAgent::delayedRemoveReplayXHR should ref before deref-ing XHR

delayedRemoveReplayXHR derefs the target XHR before adding its ref.
If the XHR ref count is 1, this may drop reference of the XHR and will ref() the dead XHR.

This CL avoids the issue by swapping its operation order.

TBR=haraken, tyoshino, vsevik
TEST=http/tests/inspector/network/network-xhr-replay.html will not crash
BUG=None

Review URL: https://codereview.chromium.org/397633002

git-svn-id: svn://svn.chromium.org/blink/trunk@178177 bbb929c8-8fbe-4397-9dbb-9b2b20218538