Call v8::Debug::SetDebugEventListener2 when removing debug event listener

We used v8::Debug::SetDebugEventListener2 to set event listener but v8::Debug::SetDebugEventListener to remove it.

BUG=None
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/19551002

git-svn-id: svn://svn.chromium.org/blink/trunk@154386 bbb929c8-8fbe-4397-9dbb-9b2b20218538