background.js

chrome.webRequest.onHeadersReceived.addListener(
  function (details) {
    for (var i = 0; i < details.responseHeaders.length; ++i) {
      if (details.responseHeaders[i].name.toLowerCase() == 'x-frame-options') {
        details.responseHeaders.splice(i, 1);
        return {
          responseHeaders: details.responseHeaders
        };
      }
    }
  }, {
    urls: ["<all_urls>"]
  }, ["blocking", "responseHeaders"]);

manifest.json

{
   "background": {
      "scripts": [ "background.js" ]
   },
   "description": "Drops X-Frame-Options HTTP response headers, allowing all pages to be iframed.",
   "key": "MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDysh7qP/8H9qUMb0R9BZbk2NDirtNgRyo9AEh9C5HFcoMELEzJ/M/sCqn/yDM/Z7GK3t+w02zCeTBknLEUwgFL9kRxVV4s1kfgbijVHTSJkII6OjqiWDPkDeyMJ1oOr156Rct2bp2MAwOf0Tk1bm2UdwNbJxoE5sQFd2Hbu+WFxQIDAQAB",
   "manifest_version": 2,
   "name": "Ignore X-Frame headers",
   "permissions": [ "webRequest", "webRequestBlocking", "\u003Call_urls>" ],
   "update_url": "http://clients2.google.com/service/update2/crx",
   "version": "1.0"
}