DevTools: Fix exception in Show Function Definition context menu for built-in functions.

We cannot jump to a native function's code - just ignore this case.
Also: refactor to match the behavior to the WebInspector.inspect().

BUG=314463
R=vsevik,pfeldman

Review URL: https://codereview.chromium.org/60163003

git-svn-id: svn://svn.chromium.org/blink/trunk@161538 bbb929c8-8fbe-4397-9dbb-9b2b20218538