Remove unused or stale SharedWorker code

This removes implementation for following methods for SharedWorker:
- postMessageToWorkerObject
- postExceptionToWorkerObject
- postConsoleMessageToWorkerObject
- confirmMessageFromWorkerObject
- reportPendingActivity

All of them are not handled in Chromium and do not seem to be
deadly necessary in SharedWorker at least in Chromium.
(E.g. messages are sent via message ports, exceptions are sent to
DevTools and pending activity doesn't make much sense in the current
shared worker architecture).

BUG=none

Review URL: https://codereview.chromium.org/34723002

git-svn-id: svn://svn.chromium.org/blink/trunk@160281 bbb929c8-8fbe-4397-9dbb-9b2b20218538