Roll third_party/polymer.

third_party/polymer/v1_0/components-chromium/iron-overlay-behavior/iron-overlay-backdrop-extracted.js

 (function() {
 
   Polymer({
 
     is: 'iron-overlay-backdrop',
 
     properties: {
 
       /**
        * Returns true if the backdrop is opened.
@@ skipping 13 matching lines @@
     },
 
     listeners: {
       'transitionend' : '_onTransitionend'
     },
 
     /**
      * Appends the backdrop to document body and sets its `z-index` to be below the latest overlay.
      */
     prepare: function() {
-      // Always update z-index
-      this.style.zIndex = this._manager.backdropZ();
       if (!this.parentNode) {
         Polymer.dom(document.body).appendChild(this);
       }
     },
 
     /**
      * Shows the backdrop if needed.
      */
     open: function() {
       // only need to make the backdrop visible if this is called by the first overlay with a backdrop
       if (this._manager.getBackdrops().length < 2) {
         this._setOpened(true);
       }
     },
 
     /**
      * Hides the backdrop if needed.
      */
     close: function() {
-      // Always update z-index
-      this.style.zIndex = this._manager.backdropZ();
       // close only if no element with backdrop is left
       if (this._manager.getBackdrops().length === 0) {
         // Read style before setting opened.
         var cs = getComputedStyle(this);
         var noAnimation = (cs.transitionDuration === '0s' || cs.opacity == 0);
         this._setOpened(false);
         // In case of no animations, complete
         if (noAnimation) {
           this.complete();
         }
@@ skipping 12 matching lines @@
 
     _onTransitionend: function (event) {
       if (event && event.target === this) {
         this.complete();
       }
     }
 
   });
 
 })();