Implement new heartbeat code to catch runaway execution of document
ECMAScript code. The old code uses JS_SetBranchCallback which is
deprecated in new versions of SpiderMonkey. The new code uses setitimer(2)
and the JS_SetOperationCallback and JS_TriggerOperationCallback interfaces,
introduced in SpiderMonkey 1.8.1. Compatibility with both the old
JS_SetBranchCallback and the new interfaces is maintained.
