IndexedDB: Prevent terminated worker from hanging connections

IndexedDB database connection shutdown is normally blocked on waiting
for transactions to finish. And transaction finishing requires an
abort/onAbort message round-trip to the back-end. When a Worker
context is stopped, the communication channel goes away before the
response comes back, so the transaction never finishes and the
database connection never closes.

Fix by explicitly closing the connection when the execution context is
stopped.

R=alecflett@chromium.org,dgrogan@chromium.org,cmumford@chromium.org
BUG=242115

Review URL: https://codereview.chromium.org/138473004

git-svn-id: svn://svn.chromium.org/blink/trunk@165179 bbb929c8-8fbe-4397-9dbb-9b2b20218538