DevTools: Fix tainted Math methods could crash the debugger.

Follow-up to Blink r172049.

BUG=364470
R=pfeldman

Review URL: https://codereview.chromium.org/247003002

git-svn-id: svn://svn.chromium.org/blink/trunk@172162 bbb929c8-8fbe-4397-9dbb-9b2b20218538