Make window.navigator property non replaceable

Make window.navigator property non replaceable non replaceable to match
the specification:
http://www.whatwg.org/specs/web-apps/current-work/multipage/browsers.html#window

The new behavior also matches the behavior of Firefox 28 and IE11.

R=arv@chromium.org, tkent@chromium.org
BUG=364512
TEST=fast/dom/Window/get-set-properties.html

Review URL: https://codereview.chromium.org/243203002

git-svn-id: svn://svn.chromium.org/blink/trunk@172091 bbb929c8-8fbe-4397-9dbb-9b2b20218538