Remove script handlers on shutdown.

This avoids a DCHECK.

Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: If764659a3b0d12955e79d155f1408e44db10277d
Reviewed-on: https://chromium-review.googlesource.com/1129962
Commit-Queue: John Wu <jzw@chromium.org>
Reviewed-by: Hiroshi Ichikawa <ichikawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574185}

ios/web_view/internal/cwv_web_view.mm

@@ -252,6 +252,10 @@ static NSString* gUserAgentProduct = nil;
 - (void)webStateDestroyed:(web::WebState*)webState {
   webState->RemoveObserver(_webStateObserver.get());
   _webStateObserver.reset();
+  for (const auto& pair : _scriptCommandCallbacks) {
+    webState->RemoveScriptCommandCallback(pair.first);
+  }
+  _scriptCommandCallbacks.clear();
 }
 
 - (void)webState:(web::WebState*)webState