Don't check resourceLoadTiming when fetched via a ServiceWorker in Performance::addResourceTiming().

Currently ServiceWorker doesn't support TimingInfo.
So ASSERT(finalTiming) failes.

BUG=373120

Review URL: https://codereview.chromium.org/396733007

git-svn-id: svn://svn.chromium.org/blink/trunk@178630 bbb929c8-8fbe-4397-9dbb-9b2b20218538