Fix no-op check to work with Safari (mozilla#364)

Verify the object that might be at `globalThis.browser` is not already
implementing the basic Web Extension APIs. This is symmetric with the
check to verify the poly fill is only included in extension contexts.

src/browser-polyfill.js

@@ -10,7 +10,7 @@ if (!globalThis.chrome?.runtime?.id) {
   throw new Error("This script should only be loaded in a browser extension.");
 }
 
-if (typeof globalThis.browser === "undefined" || Object.getPrototypeOf(globalThis.browser) !== Object.prototype) {
+if (!globalThis.browser?.runtime?.id) {
   const CHROME_SEND_MESSAGE_CALLBACK_NO_RESPONSE_MESSAGE = "The message port closed before a response was received.";
 
   // Wrapping the bulk of this polyfill in a one-time-use function is a minor