Service Worker: Fallback to network when SW interception unexpectedly fails.
Failing to dispatch the fetch event is an exceptional case: the SW may
be unexpectedly missing or it failed to start up, etc. For a main resource
load, responding with error just blocks access to the page, so try to
fallback to network in that case. For a subresource load, it'd be strange
for some subresources to come from the SW and some from the network, so
return error as currently implemented.
BUG=448003
Review URL: https://codereview.chromium.org/
1134483003
Cr-Commit-Position: refs/heads/master@{#328942}