DevTools: Avoid side effects from calling obj.constructor.name in InjectedScript.

BUG=362882, 321349
R=pfeldman

Review URL: https://codereview.chromium.org/382473002

git-svn-id: svn://svn.chromium.org/blink/trunk@177881 bbb929c8-8fbe-4397-9dbb-9b2b20218538