Verify that ExtendableMessageEvent's message ports reside on the same heap

To potentially help diagnose a GC crash (see associated bug), verify
that ExtendableMessageEvent's MessagePortArray and MessagePorts belong
to the same thread heap as the event object.

R=haraken
BUG=655926

Review-Url: https://codereview.chromium.org/2589333005
Cr-Commit-Position: refs/heads/master@{#440390}
2 files changed