Add aura::Window::GeWeakPtrAsWindow()

This CL moves the WeakPtrFactory from GestureConsumer to subclasses,
and define WeakPtr getter for Window type, so that we can use the
same factory instance for super class and base casses, but still
use WeakPtr<aura::Window> but not WeakPtr<ui::GestureConsumer>.

Background:
There are increasing report w.r.t. dangling ptr for a window that can
be destroyed during a method call. This is to simplify the code
to deal with such situation (e.g. using WindowTracker or observers).

The weak pointer factory should be the last element in the class
structure, and there is no clean way to cast base::WeakPtr<Super>
to base::WeakPtr<Sub>.

C++ doesn't support covariance for template, so we can't just
define base::WeakPtr<aura::Window> GetWeakPtr.

And replced WindowTracker that tracks single window with Weakptr.

Bug: None

Change-Id: Ifc993908f56d34e3b694a9d10e26974097923e7c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7846473
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Keren Zhu <kerenzhu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1630665}