Install per-context enabled properties on Window after associating with the wrapper.

LocalDOMWindow::document() and toDocument(context) don't match up yet at this time
of the call, so initialize the per-context enabled properties only after associating
the LocalDOMWindow with the window wrapper.

BUG=398446

Review URL: https://codereview.chromium.org/447463002

git-svn-id: svn://svn.chromium.org/blink/trunk@179551 bbb929c8-8fbe-4397-9dbb-9b2b20218538