[WebUITabStrip] Crash @TabDragDropDelegate::ShouldPreventSnapToTheEdge()

When performing a tab dragging, with WebUITabStrip ON, an instance
of TabDragDropDelegate is created. The logic in this class relies
heavily on a valid |source_window_| class member, passed in as ctor
parameter.
When, during a drag drop operation, this |source_window_ object
is destroyed, UAF crashes can happen in many places in
TabDragDropDelegate.

This CL makes use of a WindowObserver installed in DragDropController
to cease the operation of its TabDragDropDelegate instance, when
the respective |source_window_| is destroyed.

BUG=1286203
R=oshima@chromium.org

Change-Id: I3d59017945d5b3e01eaf31de7d38b2898e660d62
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3747280
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/main@{#1022261}

• ash/drag_drop/drag_drop_controller.cc[diff]
• ash/drag_drop/drag_drop_controller_unittest.cc[diff]
• ash/drag_drop/tab_drag_drop_delegate.cc[diff]