Find breakpoint by PC rather than source position when clearing breakpoint

Current approach is to find breakpoint by the statement position that was used when setting breakpoint. This doesn't work when setting breakpoint by anything else but statement position.

(Question: could PC of existing breakpoint change, for example because of recompilation, or this approach is safe)

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/15685010

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15021 ce2b1a6d-e550-0410-aec6-3dcde31c8c00