Fix memory leak in WorkerAllowMainThreadBridgeBase.

WorkerAllowMainThreadBridgeBase allocates m_workerGlobalScopeObserver, but does
not free it. This CL fixes it.

Review URL: https://codereview.chromium.org/26796002

git-svn-id: svn://svn.chromium.org/blink/trunk@159487 bbb929c8-8fbe-4397-9dbb-9b2b20218538