Reset the path tracer completely, and check whether an object has source code before trying to print

BUG=none
TEST=none

Review URL: https://chromiumcodereview.appspot.com/9749008
Patch from Jochen Eisigner <jochen@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11822 ce2b1a6d-e550-0410-aec6-3dcde31c8c00