Remove calls to deprecated V8 APIs in ScriptDebugServer.cpp

BUG=324225
R=haraken@chromium.org

Review URL: https://codereview.chromium.org/95023002

git-svn-id: svn://svn.chromium.org/blink/trunk@162839 bbb929c8-8fbe-4397-9dbb-9b2b20218538