Follow up to fix v8::Exception::GetMessage() actually do what it was intended to.

The main thing for v8::Exception::GetMessage() is to extract message location from
error stack trace, even when stack trace capturing is off (when DevTools is closed).

BUG=chromium:427954
R=yangguo@chromium.org
LOG=N

Review URL: https://codereview.chromium.org/696703002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25101 ce2b1a6d-e550-0410-aec6-3dcde31c8c00