Remove unneeded files from third_party/polymer.

third_party/polymer/v1_0/components-chromium/iron-dropdown/README.md

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-iron-dropdown.html
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-[![Build status](https://travis-ci.org/PolymerElements/iron-dropdown.svg?branch=master)](https://travis-ci.org/PolymerElements/iron-dropdown)
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-_[Demo and API docs](https://elements.polymer-project.org/elements/iron-dropdown)_
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-##&lt;iron-dropdown&gt;
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-`<iron-dropdown>` is a generalized element that is useful when you have
-hidden content (`.dropdown-content`) that is revealed due to some change in
-state that should cause it to do so.
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-Note that this is a low-level element intended to be used as part of other
-composite elements that cause dropdowns to be revealed.
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-Examples of elements that might be implemented using an `iron-dropdown`
-include comboboxes, menubuttons, selects. The list goes on.
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-The `<iron-dropdown>` element exposes attributes that allow the position
-of the `.dropdown-content` relative to the `.dropdown-trigger` to be
-configured.
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-```html
-<iron-dropdown horizontal-align="right" vertical-align="top">
-  <div class="dropdown-content">Hello!</div>
-</iron-dropdown>
-```
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-In the above example, the `<div>` with class `.dropdown-content` will be
-hidden until the dropdown element has `opened` set to true, or when the `open`
-method is called on the element.
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