Verify that FetchEvent::m_request holds same-thread reference.

To diagnose instability seen during GCs of FetchEvents, instrument
the m_request member, checking that we only create event object containing
references to Requests that reside in the current thread's heap.

R=
BUG=655926

Review-Url: https://codereview.chromium.org/2594423002
Cr-Commit-Position: refs/heads/master@{#440475}
1 file changed