back_gesture: Add a test to guard destroying window during drag

Adding a test to make sure no crash happens if the window being
destroyed while dragging the back gesture affordance from the left.
OnTouchEvent(ui::TouchEvent* event) should always been triggered with a
valid event target.
SendBackEvent(const gfx::Point& screen_location) is able to handle the
case if the window has been destroyed.

Bug: 1159634
Change-Id: I4cf9612642f53c930b6570724565e2c6c6b637f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3004171


Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Min Chen <minch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#898409}