In Chrome several tabs may share the same v8 instance. Each of those tabs can be inspected with its own devtools window. When such devtools window closes we need to clear all breakpoints that belong to it. Notion of breakpoint group enables us to do that.

Storing the breakpoints on the client(in a different process) would be inreliable since 1) it may crash and 2) some 'setbreakpoint' requests may be not processed yet by the time devtools window closes.

Review URL: http://codereview.chromium.org/149197

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2376 ce2b1a6d-e550-0410-aec6-3dcde31c8c00