Add the ability to have the host embedding V8 receive a callback in the V8 thread while V8 is processing a debug callback. When V8 is processing a debug callback the thread where V8 is executing is sitting in a tight loop processing debug commands until the continue command has been executed. In some embedding situations it is beneficial to be able to call back into the embedding host from the thread where V8 is sitting. The might have functions which needs to be called to complement the JavaScript debugging.

Using the uint16_t array to pass a void* is a temporary hack.
Review URL: http://codereview.chromium.org/20491

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1318 ce2b1a6d-e550-0410-aec6-3dcde31c8c00