views: don't dangle mouse press handler on view destruction

View::SetMouseAndGestureHandler()'s documentation alleges:

  // Note: if the mouse handler is no longer connected to a
  // view hierarchy, events won't be sent.

but actually this is not true. There is logic in
RootView::ViewHierarchyChanged() which clears mouse_pressed_handler_ if
that view is removed from the RootView's tree, but that logic *doesn't*
trigger if the handler was explicitly set. That means that doing this:

  View* v = root->AddChildView(std::make_unique<View>());
  root->SetMouseAndGestureHandler(v);
  root->RemoveChildViewT(v);

will leave root with a dangling mouse_pressed_handler_. If another mouse
event then arrives, it will be sent to that dangling pointer. It's not
easy to trigger this in production but there is a unit test
(RootViewTest.EventHandlersResetWhenDeleted) that trips the dangling
pointer detector in a similar way.

To fix that problem, have RootView clear the handler *even if it was
explicitly set* when it is removed from the RootView's view hierarchy.

Bug: 361651894
Change-Id: I75fe49e2c8439b3986a5937ed887f4334fda1a2d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5906230
Commit-Queue: Elly FJ <ellyjones@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1365015}