Add workerGlobalScopeStarted() method to WorkerReportingProxy

So that proxy implementers can react when necessary.
(We want to use this for ServiceWorker)

Also rename some WorkerReportingProxy methods:
- postConsoleMessageToWorkerObject -> reportConsoleMessage
- postExceptionToWorkerObject -> reportException

Since the target may not always be the worker object.

BUG=none
R=abarth

Review URL: https://codereview.chromium.org/77783002

git-svn-id: svn://svn.chromium.org/blink/trunk@162386 bbb929c8-8fbe-4397-9dbb-9b2b20218538