DevTools: Extract inspector specific methods from WorkerMessagingProxy to WorkerInspectorProxy.

WorkerMessagingProxy is now aggregating an WorkerInspectorProxy instance.
This will allow us to reuse the same WorkerInspectorProxy implementation for shared/service workers.

BUG=404355
R=pfeldman, yurys, sergeyv

Review URL: https://codereview.chromium.org/482493002

git-svn-id: svn://svn.chromium.org/blink/trunk@180538 bbb929c8-8fbe-4397-9dbb-9b2b20218538