Use Document of callingWindow for access check in isInsecureScriptAccess()

      The Document call was changed to Frame::securityContext() to make the code more
RemoteFrame-friendly. However, callingWindow is guaranteed local. Revert to
getting the SecurityOrigin via Document.

BUG=524074
TEST=http/tests/security/location-change-from-detached-DOMWindow.html

Review URL: https://codereview.chromium.org/1311253005

git-svn-id: svn://svn.chromium.org/blink/trunk@201139 bbb929c8-8fbe-4397-9dbb-9b2b20218538