1) Let SaveContext remember the top JS frame stack pointer so it works in simulator where C stack and JS stack are separated;
2) Use the new calling convension in %DebugBreakCallHelper function;

Review URL: http://codereview.chromium.org/10663

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@774 ce2b1a6d-e550-0410-aec6-3dcde31c8c00