DevTools: Extract WorkerInspectorProxy interface from WorkerGlobalScopeProxy.

WorkerGlobalScopeProxy is dedicated worker specific, while this interface could be reused for
shared/service workers as well.

BUG=404355
R=pfeldman, yurys, sergeyv

Review URL: https://codereview.chromium.org/477313002

git-svn-id: svn://svn.chromium.org/blink/trunk@180441 bbb929c8-8fbe-4397-9dbb-9b2b20218538