[WebUITabStrip] Crash @TabDragDropDelegate::~TabDragDropDelegate()

During an ongoing tab dragging on Lacros, with both WebUITabStrip
and TabletMode features ON, Lacros crashes in case the dragged
`exo::ShellSurface` (ie browser window) is deleted by JavaScript.

What happens is that during a tab drag 'n drop, exo::TabDragDropDelegate
store an `aura::Window` instance, and operates over it throughout its
life cycle. However, during the drag operation, it is possible that this
`aura::Window` instance gets deleted, and the pointer becomes dangling.

Here is the pseudo stacktrace of the dragged `aura::Window` instance
(and its respective exo::ShellSurface) being destroyed:

  #4  ~Window () at ../../ui/aura/window.cc:201
  #5  aura::Window::~Window() () at ../../ui/aura/window.cc:186
  #6  CloseNow () at ../../ui/views/widget/widget.cc:787
  #7  ~ShellSurfaceBase () at ../../components/exo/shell_surface_base.cc:347
  #8  ~ShellSurface () at ../../components/exo/shell_surface.cc:140
  #9  ~XdgShellSurface () at ../../components/exo/xdg_shell_surface.cc:27
  #10 ~XdgShellSurface () at ../../components/exo/xdg_shell_surface.cc:27
  #11 operator() () at (...)
  #12 reset () at (...)
  #13 ~unique_ptr () at (...)
  #14 ~WaylandXdgSurface () at ../../components/exo/wayland/xdg_shell.cc:783
  #15 operator() () at (...)
  #16 reset () at (...)
  #17 ~unique_ptr () at (...)
  #18 DestroyUserData<exo::wayland::WaylandXdgSurface> () at ../../components/exo/wayland/server_util.h:40
  #19 0x0000556095b8ae10 in destroy_resource () at ../../third_party/wayland/src/src/wayland-server.c:733
  #20 0x0000556095b8acb0 in wl_resource_destroy () at ../../third_party/wayland/src/src/wayland-server.c:750
  #21 0x00005560a6dc1d65 in ffi_call_unix64 ()
  #22 0x00005560a6dc12b9 in ffi_call_int ()

... and here is the stack trace of the crash, where this `aura::Window` is
dereferenced:

  #0  is_destroying ui/aura/window.h:166:39
  #1  ash::TabDragDropDelegate::~TabDragDropDelegate() ash/drag_drop/tab_drag_drop_delegate.cc:117:23
  #2  ash::TabDragDropDelegate::~TabDragDropDelegate() ash/drag_drop/tab_drag_drop_delegate.cc:114:45
  #3  operator() (...)
  #4  reset (...)
  #5  ash::DragDropController::Cleanup() ash/drag_drop/drag_drop_controller.cc:800:27
  #6  ash::DragDropController::DoDragCancel(base::TimeDelta) ash/drag_drop/drag_drop_controller.cc:702:3
  #7  exo::DragDropOperation::~DragDropOperation() components/exo/drag_drop_operation.cc:277:28
  #8  exo::DragDropOperation::~DragDropOperation() components/exo/drag_drop_operation.cc:270:41
  #9  exo::Surface::~Surface() components/exo/surface.cc:286:14
  #10 operator() (...)
  #11 reset (...)
  #12 ~unique_ptr (...)

This CL protects `exo::TabDragDropDelegate` dtor against this crash
by making use of `aura::WindowObserver` logic and null-checking
`TabDragDropDelegate::source_window_`.

Last, this CL also extends exo::ExoTestBase class with an
additional SetUp() method, that takes a TestShellDelegate as
parameter.

BUG=1348791

Change-Id: Ic00886fda66d4150ce07d74d6aaccf31e77bc80b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4174397
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/main@{#1094249}

• ash/drag_drop/tab_drag_drop_delegate.cc[diff]
• ash/drag_drop/tab_drag_drop_delegate.h[diff]
• components/exo/drag_drop_operation_unittest.cc[diff]
• components/exo/test/exo_test_base.cc[diff]
• components/exo/test/exo_test_base.h[diff]