Fix crashes after r167276

r167276 hits the following assertion:

  STDERR: ../../third_party/WebKit/Source/bindings/v8/DOMWrapperWorld.cpp(80) : static WebCore::DOMWrapperWorld *WebCore::DOMWrapperWorld::mainWorld()

The cause is that r167276 replaced toDOMWindow with contextHasCorrectPrototype.
This CL reverts the replacement.

TBR=jochen,dcarney

Review URL: https://codereview.chromium.org/169193003

git-svn-id: svn://svn.chromium.org/blink/trunk@167289 bbb929c8-8fbe-4397-9dbb-9b2b20218538