code: function injectAddons(manifestUrls) {
    var addons = {};
    for (var n = 0; n < manifestUrls.length; n++) {
      // Fetch Manifest JSON.
      var url = manifestUrls[n];
      var json = getUrl(url);
      if (!json) {
        log('Unable to fetch manifest JSON: ' + url);
        continue;
      }
      json = JSON.parse(json);
      addons[url] = json;
  
      // If it has a tracing node, inject scripts.
      var tracingInfo = json['tracing'];
      if (tracingInfo && tracingInfo['scripts']) {
        var tracingScripts = tracingInfo['scripts'];
        for (var m = 0; m < tracingScripts.length; m++) {
          var scriptUrl = resolveUrl(url, tracingScripts[m]);
          if (!injectScriptFile(scriptUrl)) {
            log('Error loading addon ' + url + ':');
            log('Tracing script file not found: ' + scriptUrl);
          }
        }
      }
    }
    return addons;
  }
docs: Injects the given addons into the page.
  @param {!Array.<string>} manifestUrls Addon manifest JSON URLs.
  @return {!Object.<!Object>} Addon manifests, mapped by manifest URL.