Roll Polymer elements

third_party/polymer/v1_0/components-chromium/app-layout/app-scroll-effects/effects/blend-background-extracted.js

+/**
+   * While scrolling down, fade in the rear background layer and fade out the front background
+   * layer (opacity interpolated based on scroll position).
+   */
+  Polymer.AppLayout.registerEffect('blend-background', {
+    /** @this Polymer.AppLayout.ElementWithBackground */
+    setUp: function setUp() {
+      this.$.backgroundFrontLayer.style.willChange = 'opacity';
+      this.$.backgroundFrontLayer.style.webkitTransform = 'translateZ(0)';
+      this.$.backgroundRearLayer.style.willChange = 'opacity';
+      this.$.backgroundRearLayer.style.webkitTransform = 'translateZ(0)';
+      this.$.backgroundRearLayer.style.opacity = 0;
+    },
+    /** @this Polymer.AppLayout.ElementWithBackground */
+    run: function run(p, y) {
+      this.$.backgroundFrontLayer.style.opacity = 1 - p;
+      this.$.backgroundRearLayer.style.opacity = p;
+    }
+  });