Move WorkerThreadLifecycleObserver::contextDestroyed out of SafePointScope

Oilpan forbids to allocate objects in a SafePointScope. It is too restrictive
for WorkerThreadLifecycleObserver::contextDestroyed, so this CL moves the
WorkerThreadLifecycleContext::notifyContextDestroyed call out of
SafePointScope.

BUG=587663

Review-Url: https://codereview.chromium.org/2161923002
Cr-Commit-Position: refs/heads/master@{#406275}
1 file changed