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After processing a batch of requests, our IDB transaction initiates an
inactivity timer; if the timer fires before any further requests
arrive from the front-end we assume the front-end is wedged and abort
the transaction. The timer is be stopped on any new request, including
a signal to commit or abort. During the introduction of blobs, the
"stop timer on commit signal" ended towards the end of the commit
flow, but it should be towards the start so it doesn't fire if the
back-end is busy.

Speculative fix for frequent timeouts of W3C web-platform-tests seen
on slow bots.

R=cmumford@chromium.org

Review URL: https://codereview.chromium.org/502643003

Cr-Commit-Position: refs/heads/master@{#292035}