Add root element definition

Instead of appending/removing elements to document.body, add a global
that defines what the root element is. This way, in apps that use
elements with addional content outside, they will just need to set a
root element.

Change-Id: I0d01e4a6eea7d7d0af39989f1f831832506f1774

polygerrit-ui/app/behaviors/gr-tooltip-behavior/gr-tooltip-behavior.html

@@ -15,4 +15,6 @@ limitations under the License.
 -->
 <link rel="import" href="../../bower_components/polymer/polymer.html">
 <link rel="import" href="../../elements/shared/gr-tooltip/gr-tooltip.html">
+<script src="../../scripts/rootElement.js"></script>
+
 <script src="gr-tooltip-behavior.js"></script>

polygerrit-ui/app/behaviors/gr-tooltip-behavior/gr-tooltip-behavior.js

@@ -75,7 +75,7 @@
       // Set visibility to hidden before appending to the DOM so that
       // calculations can be made based on the element’s size.
       tooltip.style.visibility = 'hidden';
-      Polymer.dom(document.body).appendChild(tooltip);
+      Gerrit.getRootElement().appendChild(tooltip);
       this._positionTooltip(tooltip);
       tooltip.style.visibility = null;
 

polygerrit-ui/app/elements/shared/gr-alert/gr-alert.html

@@ -17,6 +17,8 @@ limitations under the License.
 <link rel="import" href="../../../bower_components/polymer/polymer.html">
 <link rel="import" href="../gr-button/gr-button.html">
 
+<script src="../../../scripts/rootElement.js"></script>
+
 <dom-module id="gr-alert">
   <template>
     <style>

polygerrit-ui/app/elements/shared/gr-alert/gr-alert.js

@@ -60,14 +60,14 @@
       this.actionText = opt_actionText;
       this._hideActionButton = !opt_actionText;
       this._actionCallback = opt_actionCallback;
-      document.body.appendChild(this);
+      Gerrit.getRootElement().appendChild(this);
       this._setShown(true);
     },
 
     hide: function() {
       this._setShown(false);
       if (this._hasZeroTransitionDuration()) {
-        document.body.removeChild(this);
+        Gerrit.getRootElement().removeChild(this);
       }
     },
 
@@ -81,7 +81,7 @@
     _handleTransitionEnd: function(e) {
       if (this.shown) { return; }
 
-      document.body.removeChild(this);
+      Gerrit.getRootElement().removeChild(this);
     },
 
     _handleActionTap: function(e) {

polygerrit-ui/app/scripts/rootElement.js

@@ -0,0 +1,20 @@
+// Copyright (C) 2017 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+(function(window) {
+  window.Gerrit = window.Gerrit || {};
+  if (window.Gerrit.hasOwnProperty('getRootElement')) { return; }
+
+  window.Gerrit.getRootElement = () => document.body;
+})(window);