Bug 1254385 - Apply dPR from selected device; r=jryans

- dPR and media queries are now reflecting the selected device's dppx
- added unit test

MozReview-Commit-ID: 9634mleTwkS

devtools/client/responsive.html/components/browser.js

@@ -102,6 +102,7 @@ module.exports = createClass({
 
   stopFrameScript: Task.async(function* () {
     let { onContentResize } = this;
+
     let browser = this.refs.browserContainer.querySelector("iframe.browser");
     let mm = browser.frameLoader.messageManager;
     e10s.off(mm, "OnContentResize", onContentResize);