Respond to the first port request, ignore additional ones

On Chrome, transferring the port more than once causes an error:

```
const {port1, port2} = new MessageChannel();
window.postMessage('hello', '*', [port1])
window.postMessage('hello', '*', [port1])
> Uncaught DOMException: Failed to execute 'postMessage' on 'Window': Port at index 0 is already neutered.
    at <anonymous>:1:8
```

As a result of `PortFinder` polling the host, `PortProvider` could
respond multiple times, specially when the polling frequency is high.

src/shared/port-provider.js

@@ -86,10 +86,6 @@ export class PortProvider {
     // Create the `host-sidebar` channel immediately, while other channels are
     // created on demand
     this._hostSidebarChannel = new MessageChannel();
-    this._channels.set(
-      'host-sidebar',
-      new Map([[window, this._hostSidebarChannel]])
-    );
 
     this._listeners = new ListenerCollection();
     this._listen();
@@ -233,17 +229,7 @@ export class PortProvider {
       return;
     }
 
-    const { channel, port } = options.allowedMessage;
-
-    // Check if channel has already been created. `host-sidebar` channel is an
-    // special case, because is created in the constructor.
-    if (channel === 'host-sidebar' && port === 'sidebar') {
-      this._sendPort(event, {
-        ...options,
-        port: this._hostSidebarChannel.port2,
-      });
-      return;
-    }
+    const { channel } = options.allowedMessage;
 
     let windowChannelMap = this._channels.get(channel);
     if (!windowChannelMap) {
@@ -254,10 +240,25 @@ export class PortProvider {
     const source = /** @type {Window} */ (event.source);
     let messageChannel = windowChannelMap.get(source);
 
-    if (!messageChannel) {
+    // Ignore the port request if the channel for the specified window has
+    // already been created. This is to avoid transfer the port more than once.
+    if (messageChannel) {
+      return;
+    }
+
+    // `host-sidebar` channel is an special case, because it is created in the
+    // constructor.
+    if (channel === 'host-sidebar') {
+      windowChannelMap.set(source, this._hostSidebarChannel);
+      this._sendPort(event, {
+        ...options,
+        port: this._hostSidebarChannel.port2,
+      });
+      return;
+    }
+
     messageChannel = new MessageChannel();
     windowChannelMap.set(source, messageChannel);
-    }
 
     const { port1, port2 } = messageChannel;
     this._sendPort(event, { ...options, port: port1, reciprocalPort: port2 });

src/shared/test/port-provider-test.js

@@ -159,6 +159,29 @@ describe('PortProvider', () => {
       );
     });
 
+    it('responds to the first valid port request, ignore additional requests', async () => {
+      const data = {
+        channel: 'host-sidebar',
+        port: 'sidebar',
+        source,
+        type: 'request',
+      };
+
+      for (let i = 0; i < 4; ++i) {
+        sendMessage({
+          data,
+        });
+      }
+      await delay(0);
+
+      assert.calledOnceWithExactly(
+        window.postMessage,
+        { ...data, type: 'offer' },
+        window.location.origin,
+        [sinon.match.instanceOf(MessagePort)]
+      );
+    });
+
     it('sends the reciprocal port of the `guest-sidebar` channel (via the sidebar port)', async () => {
       sendMessage({
         data: {