fix rrweb-io#469 try to get original MutationObserver

We found Angular's zone module will patch MutationObserver which
make the browser hang in some scenarios.
Reference: angular/angular#26948

src/record/observer.ts

@@ -44,6 +44,12 @@ import {
 import MutationBuffer from './mutation';
 import { stringify } from './stringify';
 
+type WindowWithAngularZone = Window & {
+  Zone?: {
+    __symbol__?: (key: string) => string;
+  };
+};
+
 export const mutationBuffer = new MutationBuffer();
 
 function initMutationObserver(
@@ -65,7 +71,22 @@ function initMutationObserver(
     recordCanvas,
     slimDOMOptions,
   );
-  const observer = new MutationObserver(
+  let mutationBufferCtor = window.MutationObserver;
+  const angularZoneSymbol = (window as WindowWithAngularZone)?.Zone?.__symbol__?.(
+    'MutationObserver',
+  );
+  if (
+    angularZoneSymbol &&
+    ((window as unknown) as Record<string, typeof MutationObserver>)[
+      angularZoneSymbol
+    ]
+  ) {
+    mutationBufferCtor = ((window as unknown) as Record<
+      string,
+      typeof MutationObserver
+    >)[angularZoneSymbol];
+  }
+  const observer = new mutationBufferCtor(
     mutationBuffer.processMutations.bind(mutationBuffer),
   );
   observer.observe(document, {