Replace chrome.extension with chrome.runtime

The messaging APIs in `extension` module are deprecated in favor of
the equivalent APIs in the `runtime` module.

Sources:
https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/extension
https://developer.chrome.com/extensions/extension#methods

inject.js

@@ -1,4 +1,4 @@
-chrome.extension.sendMessage({}, function(response) {
+chrome.runtime.sendMessage({}, function(response) {
   var tc = {
     settings: {
       speed: 1.0,           // default 1x
@@ -117,7 +117,7 @@ chrome.extension.sendMessage({}, function(response) {
       var shadow = wrapper.createShadowRoot();
       var shadowTemplate = `
         <style>
-          @import "${chrome.extension.getURL('shadow.css')}";
+          @import "${chrome.runtime.getURL('shadow.css')}";
         </style>
 
         <div id="controller" style="top:${top}; left:${left}">
@@ -213,7 +213,7 @@ chrome.extension.sendMessage({}, function(response) {
         defineVideoController();
       } else {
         var link = document.createElement('link');
-        link.href = chrome.extension.getURL('inject.css');
+        link.href = chrome.runtime.getURL('inject.css');
         link.type = 'text/css';
         link.rel = 'stylesheet';
         document.head.appendChild(link);

popup.js

@@ -1,6 +1,6 @@
 document.addEventListener('DOMContentLoaded', function () {
   document.querySelector('#config').addEventListener('click', function() {
-    window.open(chrome.extension.getURL("options.html"));
+    window.open(chrome.runtime.getURL("options.html"));
   });
   
   document.querySelector('#about').addEventListener('click', function() {