DevTools: Protect InjectedScript from overriding toString method.

BUG=427827
R=yurys, pfeldman

Review URL: https://codereview.chromium.org/685133002

git-svn-id: svn://svn.chromium.org/blink/trunk@184583 bbb929c8-8fbe-4397-9dbb-9b2b20218538