Add 'error' property to exceptions thrown via Worker::importScripts.

If an imported script throws an exception, and the script is same-origin
with the Worker, we should provide the exception object when handling
the exception via 'self.onerror'. The exception should not, however, be
available to the page-level 'window.onerror' (we'd be leaking state
across worlds otherwise (also, the spec says not to[1])).

This patch pulls the logic governing the exception object's storage
(wrapping the ErrorEvent, storing the value on a hidden property) out of
V8Initializer, and moves it to a static method on V8ErrorHandler so that
it's easily reusable inside WorkerScriptController.

[1]: http://www.whatwg.org/specs/web-apps/current-work/multipage/workers.html#runtime-script-errors-0

BUG=270005

Review URL: https://chromiumcodereview.appspot.com/22650008

git-svn-id: svn://svn.chromium.org/blink/trunk@155953 bbb929c8-8fbe-4397-9dbb-9b2b20218538