[M144-LTS] gtk: Fix Use-After-Free in GtkUiPlatformWayland::OnHandleSetTransient

GtkUiPlatformWayland::SetGtkWidgetTransientFor binds a raw GtkWidget*
to an asynchronous Wayland callback. If the dialog is closed before
the callback executes, the GtkWidget is destroyed, leading to a
Use-After-Free when the callback dereferences it.

This CL fixes the issue by using ScopedGObject to hold a strong
reference to the GtkWidget in the callback. This ensures the widget's
memory remains valid until the callback completes, even if it has
been destroyed/disposed by GTK.

R=thestig

(cherry picked from commit eed57ee58f276d40af4c842d7d900f68482cc5f1)

Change-Id: I402b18c246c9192038418f1348ff9066089917eb
Fixed: 500033878
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7737624
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Cr-Original-Commit-Position: refs/heads/main@{#1611064}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7902116
Reviewed-by: Mohamed Omar <mohamedaomar@google.com>
Owners-Override: Mohamed Omar <mohamedaomar@google.com>
Cr-Commit-Position: refs/branch-heads/7559@{#4995}
Cr-Branched-From: 223dfbac1c7542a06b422390d954afe5b560b607-refs/heads/main@{#1552494}