Null check the finalTiming ResourceLoadTiming* in Performance::addResourceTiming()

We've been seeing a bunch of crashes because of it being null.

BUG=263855

Review URL: https://chromiumcodereview.appspot.com/19700008

git-svn-id: svn://svn.chromium.org/blink/trunk@154855 bbb929c8-8fbe-4397-9dbb-9b2b20218538