loadSelectedContent_() {
    return new goog.Promise(function(resolve, reject) {
      var origin = this.getContent().origin;
      var textArea = /** @type {HTMLTextAreaElement} */
          (this.getElement().querySelector('textarea'));
      var content = this.getContent().selection || '';
      var detectedAction = utils.text.getPgpAction(content);
      if (detectedAction == constants.Actions.DECRYPT_VERIFY) {
        this.actionExecutor_.execute(
            /** @type {!e2e.ext.messages.ApiRequest} */ ({
              action: constants.Actions.DECRYPT_VERIFY,
              content: content,
              passphraseCallback: goog.bind(this.renderPassphraseDialog, this)
            }),
            this, goog.bind(function(decrypted) {
              if (e2e.openpgp.asciiArmor.isDraft(content)) {
                textArea.value = decrypted;
              } else {
                this.renderReply_(textArea, decrypted);
              }
              resolve();
            }, this), goog.bind(function(error) {
              this.errorCallback_(error);
              resolve();
            }, this));
      } else {
        resolve();
      }
    }, this);
  }
Loads the content that the user has selected in the web application.
@return {!goog.Promise} Promise resolved when content has been loaded.
    It's never rejected.
@private