WebUI: CHECK() that SetRequestFilter() is not being called 2+ times

Bug: none
Change-Id: I796e54e6d92123dde787df0a674c4ba556fcf56a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1869750
Commit-Queue: Dan Beam <dbeam@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Auto-Submit: Dan Beam <dbeam@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708060}

content/browser/webui/web_ui_data_source_impl.cc

@@ -175,6 +175,8 @@ void WebUIDataSourceImpl::SetDefaultResource(int resource_id) {
 void WebUIDataSourceImpl::SetRequestFilter(
     const ShouldHandleRequestCallback& should_handle_request_callback,
     const HandleRequestCallback& handle_request_callback) {
+  CHECK(!should_handle_request_callback_);
+  CHECK(!filter_callback_);
   should_handle_request_callback_ = should_handle_request_callback;
   filter_callback_ = handle_request_callback;
 }