webview: fix host.js logic (#10336)

The logic previously stopped the handler after running
`this.postMessage` but I forgot to follow this when modifying this code
in another PR.

Fix the logic by respecting what was previously done.

packages/plugin-ext/src/main/browser/webview/pre/host.js

@@ -26,6 +26,8 @@
         constructor() {
             this.handlers = new Map();
             window.addEventListener('message', e => {
+                // Note: `window.parent === window` when there is no parent...
+                const sourceIsSelfOrParentFrame = e.source === window.parent;
                 let sourceIsChildFrame = false;
                 for (let i = 0; i < window.frames.length; i++) {
                     const frame = window.frames[i];
@@ -35,11 +37,8 @@
                     }
                 }
                 if (sourceIsChildFrame && e.data && (e.data.command === 'onmessage' || e.data.command === 'do-update-state')) {
-                    // Came from inner iframe
                     this.postMessage(e.data.command, e.data.data);
-                }
-                // Note: `window.parent === window` when there is no parent...
-                if (sourceIsChildFrame || e.source === window.parent) {
+                } else if (sourceIsChildFrame || sourceIsSelfOrParentFrame) {
                     const channel = e.data.channel;
                     const handler = this.handlers.get(channel);
                     if (handler) {