paper-slider only respects [dir=rtl] but not direction: ltr; in CSS nor nested [dir]

### Description

`paper-slider` added RTL support via `:host-context([dir="rtl"])` in https://github.com/PolymerElements/paper-slider/pull/181

This is super, except there are a few ways of changing text direction that it doesn't detect.

For example:

```css
paper-slider {
  direction: rtl;
}
```
or
```html
<html dir="rtl">
<paper-slider dir="ltr">
```

### Expected outcome
Slider's direction and effective text direction agree.



### Actual outcome
In some cases, the slider direction differs from the text direction.



### Live Demo
https://jsfiddle.net/7qxd7cjx/


### Steps to reproduce
1. plop `<paper-slider style="direction: rtl;">` or `<div dir=rtl><paper-slider dir=ltr>` onto a page
2. slide the knob



### Browsers Affected

- [X] Chrome
- [X] Firefox
- [X] Safari 9
- [X] Safari 8
- [X] Safari 7
- [X] Edge
- [X] IE 11
- [X] IE 10

(Note: I assume this applies to all browsers but haven't actually checked).

/cc @dschuyler @keanulee

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paper-slider only respects [dir=rtl] but not direction: ltr; in CSS nor nested [dir] #190

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paper-slider only respects [dir=rtl] but not direction: ltr; in CSS nor nested [dir] #190

Description

Description

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Actual outcome

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Activity

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