DevTools: remove async operation's AsyncCallChain when the operation is complete
DevTools: remove async operation's AsyncCallChain when the operation is complete

This is a follow-up fix after r188733. Without the fix AsyncCallChain would only be removed when corresponding ExecutionContext is destroyed or async operation tracking is disabled.

BUG=439376

Review URL: https://codereview.chromium.org/849333003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@188743 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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