Implement Inspector related methods in WebEmbeddedWorkerImpl.

These method will be called from outside the worker's thread.

BUG=358657

Review URL: https://codereview.chromium.org/257753002

git-svn-id: svn://svn.chromium.org/blink/trunk@172631 bbb929c8-8fbe-4397-9dbb-9b2b20218538