Restructure shim/bridge initialization steps

ext/src/content.js

@@ -1,12 +1,37 @@
 "use strict";
 
-document.addEventListener("__castMessageResponse", ev => {
-	browser.runtime.sendMessage(ev.detail);
-})
-
-browser.runtime.onMessage.addListener(message => {
-	const event = new CustomEvent("__castMessage", {
-		detail: JSON.stringify(message)
-	});
-	document.dispatchEvent(event);
+const backgroundPort = browser.runtime.connect({
+    name: "shim"
+});
+
+backgroundPort.onMessage.addListener(message => {
+    const event = new CustomEvent("__castMessage", {
+        detail: JSON.stringify(message)
+    });
+    document.dispatchEvent(event);
+});
+
+let popupPort;
+browser.runtime.onConnect.addListener(port => {
+    if (port.name === "popup") {
+        popupPort = port;
+    }
+
+    port.onMessage.addListener(message => {
+        const event = new CustomEvent("__castMessage", {
+            detail: JSON.stringify(message)
+        });
+        document.dispatchEvent(event);
+    })
+});
+
+document.addEventListener("__castMessageResponse", ev => {
+    if (ev.detail.destination === "popup") {
+        if (popupPort) {
+            popupPort.postMessage(ev.detail);
+        }
+        return;
+    }
+
+    backgroundPort.postMessage(ev.detail);
 });