installGlass(opt_callback) {
    this.targetElem_.lookingGlass = this;
    goog.array.extend(this.targetElemChildren_, this.targetElem_.childNodes);

    var glassFrame = goog.dom.createElement(goog.dom.TagName.IFRAME);
    glassFrame.scrolling = 'no';
    goog.style.setSize(glassFrame, goog.style.getSize(this.targetElem_));
    glassFrame.style.border = 0;

    var pgpMessage =
        e2e.openpgp.asciiArmor.extractPgpBlock(this.targetElem_.innerText);
    var surroundings = this.targetElem_.innerText.split(pgpMessage);
    this.targetElem_.textContent = '';
    this.targetElem_.appendChild(document.createTextNode(surroundings[0]));
    this.targetElem_.appendChild(document.createElement('p'));
    // TODO(radi): Render in a shadow DOM.
    this.targetElem_.appendChild(glassFrame);
    this.targetElem_.appendChild(document.createElement('p'));
    this.targetElem_.appendChild(document.createTextNode(surroundings[1]));

    glassFrame.addEventListener('load', goog.bind(function() {
      glassFrame.contentWindow.postMessage(
          goog.crypt.base64.encodeString(
              pgpMessage, goog.crypt.base64.Alphabet.WEBSAFE_DOT_PADDING),
          chrome.runtime.getURL(''));
      if (opt_callback) {
        opt_callback();
      }
    }, this), false);

    glassFrame.addEventListener('mousewheel', function(evt) {
      evt.stopPropagation();
      evt.preventDefault();
    });
    // Loading the document after an onload handler has been bound.
    glassFrame.src = chrome.runtime.getURL('glass.html');
  }
Installs the looking glass into the hosting page.
@param {function()=} opt_callback Callback function to call when the glass
    frame was loaded.