DanglingPtr: annotate a dangling ptr in FileSystemOperationRunner

The triggering tests is `FileSystemChooserBrowserTest.FullscreenSaveFile`.

Bug: 1291138
Change-Id: Ic615431ee93f0b1ce2dff82e25aeb7ffdf3575ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3788158
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Commit-Queue: Paul Semel <paulsemel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1028650}

storage/browser/file_system/file_system_operation_runner.h

@@ -304,7 +304,7 @@ class COMPONENT_EXPORT(STORAGE_BROWSER) FileSystemOperationRunner {
   void FinishOperation(OperationID id);
 
   // Not owned; whatever owns this has to make sure context outlives this.
-  raw_ptr<FileSystemContext> file_system_context_;
+  raw_ptr<FileSystemContext, DanglingUntriaged> file_system_context_;
 
   using Operations =
       std::map<OperationID, std::unique_ptr<FileSystemOperation>>;