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

third_party/polymer/v1_0/components-chromium/iron-dropdown/bower.json

 {
   "name": "iron-dropdown",
-  "version": "1.4.1",
+  "version": "1.5.0",
   "description": "An unstyled element that works similarly to a native browser select",
   "authors": [
     "The Polymer Authors"
   ],
   "keywords": [
     "web-components",
     "web-component",
     "polymer"
   ],
   "main": "iron-dropdown.html",
   "private": true,
   "repository": {
     "type": "git",
     "url": "git://github.com/PolymerElements/iron-dropdown"
   },
   "license": "http://polymer.github.io/LICENSE.txt",
   "homepage": "https://github.com/PolymerElements/iron-dropdown",
   "dependencies": {
-    "polymer": "polymer/polymer#^1.0.0",
+    "polymer": "polymer/polymer#^1.1.0",
     "iron-behaviors": "polymerelements/iron-behaviors#^1.0.0",
     "iron-overlay-behavior": "polymerelements/iron-overlay-behavior#^1.0.0",
     "iron-resizable-behavior": "polymerelements/iron-resizable-behavior#^1.0.0",
     "neon-animation": "polymerelements/neon-animation#^1.0.0",
     "iron-a11y-keys-behavior": "polymerelements/iron-a11y-keys-behavior#^1.0.0"
   },
   "devDependencies": {
     "iron-component-page": "polymerelements/iron-component-page#^1.0.0",
     "test-fixture": "polymerelements/test-fixture#^1.0.0",
     "iron-test-helpers": "polymerelements/iron-test-helpers#^1.0.0",
     "paper-styles": "polymerelements/paper-styles#^1.0.0",
     "webcomponentsjs": "webcomponents/webcomponentsjs#^0.7.0",
     "web-component-tester": "^4.0.0",
     "iron-image": "polymerelements/iron-image#^1.0.0"
   },
   "ignore": []
 }