Remove unused mothods from ConsoleMessage.

These public methods are never called, so remove them.
-ConsoleMessage::source()
-ConsoleMessage::message()

BUG=none.

Review URL: https://codereview.chromium.org/123443004

git-svn-id: svn://svn.chromium.org/blink/trunk@164663 bbb929c8-8fbe-4397-9dbb-9b2b20218538