Remove DOMWrapperWorld::current from binding callbacks for window.event
Remove DOMWrapperWorld::current from binding callbacks for window.event

Currently window.event getter and setter is retrieving a global object of the target frame in a very redundant way using toV8Context(frame, DOMWrapperWorld::current()). Alternately, we can just retrieve it using info.Holder()->CreationContext->Global().

This CL doesn't change any existing behavior.

BUG=357144

Review URL: https://codereview.chromium.org/296903002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@174506 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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