service worker: Bind Mojo interfaces using the WorkerThread's task runner.

Attempted fix for the linked bugs. Based on crash reports, it appears
some Mojo calls are happening on the wrong thread or on the service
worker thread after it's started being destructed/terminated. To try
to fix this, explicitly bind the interfaces using the WorkerThread's
task runner, which seems to have guarantees about safely dropping
tasks when the worker thread starts being destructed/terminated.

Bug: 927149, 907311, 934622
Change-Id: I4d8e59ada9cd28aa22039fbbb77f5aa0d149ff28
Reviewed-on: https://chromium-review.googlesource.com/c/1496264
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Commit-Queue: Matt Falkenhagen <falken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637204}
8 files changed