[third_party] add polymer 1.2.3

polymer_1.2.3/bower_components/iron-flex-layout/README.md

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+<!---
+
+This README is automatically generated from the comments in these files:
+iron-flex-layout.html
+
+Edit those files, and our readme bot will duplicate them over here!
+Edit this file, and the bot will squash your changes :)
+
+-->
+
+[![Build Status](https://travis-ci.org/PolymerElements/iron-flex-layout.svg?branch=master)](https://travis-ci.org/PolymerElements/iron-flex-layout)
+
+_[Demo and API Docs](https://elements.polymer-project.org/elements/iron-flex-layout)_
+
+
+##&lt;iron-flex-layout&gt;
+
+
+The `<iron-flex-layout>` component provides simple ways to use [CSS flexible box layout](https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Flexible_boxes), also known as flexbox. This component provides two different ways to use flexbox:
+
+1. [Layout classes](https://github.com/PolymerElements/iron-flex-layout/tree/master/classes). The layout class stylesheet provides a simple set of class-based flexbox rules. Layout classes let you specify layout properties directly in markup.
+
+2. [Custom CSS mixins](https://github.com/PolymerElements/iron-flex-layout/blob/master/iron-flex-layout.html). The mixin stylesheet includes custom CSS mixins that can be applied inside a CSS rule using the `@apply` function.
+
+A complete [guide](https://elements.polymer-project.org/guides/flex-layout) to `<iron-flex-layout>` is available.
+
+