LeakDetector: Terminate all WebEmbeddedWorkerImpl before counting

Before this patch, the document held by ServiceWorker registered by
the LayoutTests were detected as leaks. The service worker is designed to live
longer than registerer document. The service worker implementation,
|WebEmbeddedWorkerImpl| carries a |WebFrame| to hold a resource loader.
The frame has its associating document, and it confused WebLeakDetector.

This patch work arounds the issue by terminating all |WebEmbeddedWorkerImpl|
instances before counting number of documents.

BUG=396493

Review URL: https://codereview.chromium.org/413993002

git-svn-id: svn://svn.chromium.org/blink/trunk@178996 bbb929c8-8fbe-4397-9dbb-9b2b20218538