Roll Polymer from 1.5.0 -> 1.6.0 to pick up native CSS custom props

third_party/polymer/v1_0/components-chromium/paper-menu-button/bower.json

 {
   "name": "paper-menu-button",
-  "version": "1.1.1",
+  "version": "1.4.1",
   "description": "A material design element that composes a trigger and a dropdown menu",
   "authors": [
     "The Polymer Authors"
   ],
   "keywords": [
     "web-components",
     "web-component",
     "polymer",
     "menu",
     "button"
   ],
   "main": "paper-menu-button.html",
   "private": true,
   "repository": {
     "type": "git",
     "url": "git://github.com/PolymerElements/paper-menu-button.git"
   },
   "license": "http://polymer.github.io/LICENSE.txt",
   "homepage": "https://github.com/PolymerElements/paper-menu-button",
   "dependencies": {
     "polymer": "Polymer/polymer#^1.1.0",
     "iron-a11y-keys-behavior": "PolymerElements/iron-a11y-keys-behavior#^1.0.0",
     "iron-behaviors": "PolymerElements/iron-behaviors#^1.0.0",
     "iron-dropdown": "PolymerElements/iron-dropdown#^1.0.0",
+    "iron-fit-behavior": "PolymerElements/iron-fit-behavior#^1.2.0",
     "neon-animation": "PolymerElements/neon-animation#^1.0.0",
     "paper-styles": "PolymerElements/paper-styles#^1.0.0"
   },
   "devDependencies": {
     "iron-component-page": "PolymerElements/iron-component-page#^1.0.0",
     "iron-icon": "PolymerElements/iron-icon#^1.0.0",
     "iron-icons": "PolymerElements/iron-icons#^1.0.0",
     "iron-image": "PolymerElements/iron-image#^1.0.0",
     "iron-test-helpers": "PolymerElements/iron-test-helpers#^1.0.0",
     "paper-button": "PolymerElements/paper-button#^1.0.0",
     "paper-icon-button": "PolymerElements/paper-icon-button#^1.0.0",
     "paper-item": "PolymerElements/paper-item#^1.0.0",
     "paper-menu": "PolymerElements/paper-menu#^1.0.0",
     "test-fixture": "PolymerElements/test-fixture#^1.0.0",
     "web-component-tester": "^4.0.0",
     "webcomponentsjs": "webcomponents/webcomponentsjs#^0.7.0"
   },
   "ignore": []
 }