Turn window.postMessage into an ActiveDOMObject

When JavaScript execution is paused by the debugger messages sent via postMessage should be suspended similar to other events.

BUG=224874

Review URL: https://codereview.chromium.org/157243003

git-svn-id: svn://svn.chromium.org/blink/trunk@168026 bbb929c8-8fbe-4397-9dbb-9b2b20218538