Mark CXFA_FFDocView::UpdateScope as CPPGC stack allocated.

Small safety check in amongst all these GC'd classes.

Change-Id: I2c4a1a79e292cbc28ed5497bf294d38da80fb401
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/89990
Auto-Submit: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/xfa/fxfa/cxfa_ffdocview.h b/xfa/fxfa/cxfa_ffdocview.h
index 1e9af72..a10e2c4 100644
--- a/xfa/fxfa/cxfa_ffdocview.h
+++ b/xfa/fxfa/cxfa_ffdocview.h
@@ -32,6 +32,8 @@
   enum class LayoutStatus : uint8_t { kNone, kStart, kDoing, kEnd };
 
   class UpdateScope {
+    CPPGC_STACK_ALLOCATED();
+
    public:
     explicit UpdateScope(CXFA_FFDocView* pDocView);
     ~UpdateScope();