service worker: Wait for request body blobs to finish construction
Blob construction recently became async and we have to handle that.
Note to sheriff: there's a good chance the crash test added will be
flaky on debug builds. That's because of an existing crash
crbug.com/611317 which happens before this patch. In that event,
I'd rather we add the test to TestExpectations than revert the
patch.
BUG=
608544
Committed:
https://crrev.com/8e6cf517c56051b81e32c05dd2dddb024c93ffe2
Cr-Commit-Position: refs/heads/master@{#394241}