Remove third_party/polymer from .gitignore

third_party/polymer/v1_0/components/iron-collapse/README.md

Index: third_party/polymer/v1_0/components/iron-collapse/README.md
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-# iron-collapse
-
-`iron-collapse` creates a collapsible block of content.  By default, the content
-will be collapsed.  Use `opened` or `toggle()` to show/hide the content.
-
-```html
-<button on-click="toggle">toggle collapse</button>
-
-<iron-collapse id="collapse">
-  <div>Content goes here...</div>
-</iron-collapse>
-```
-
-```javascript
-toggle: function() {
-  this.$.collapse.toggle();
-}
-```
-
-`iron-collapse` adjusts the height/width of the collapsible element to show/hide
-the content.  So avoid putting padding/margin/border on the collapsible directly,
-and instead put a div inside and style that.
-
-```html
-<style>
-  .collapse-content {
-    padding: 15px;
-    border: 1px solid #dedede;
-  }
-</style>
-
-<iron-collapse>
-  <div class="collapse-content">
-    <div>Content goes here...</div>
-  </div>
-</iron-collapse>
-```