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: svn://svn.chromium.org/blink/trunk@174506 bbb929c8-8fbe-4397-9dbb-9b2b20218538