back gesture: Fix crash when swiping on the backdrop.

According to [1], a sequence of gesture events is generated by the
gesture recognizer. These events currently all share the same event
target so if the target gets invalidated by the first event, the next
event would crash when we try coordinate conversion using the dangling
pointer.

This can happen if the backdrop is visible with a non-resizable app. On
swipe/fling we'll get a sequence of SCROLL_END -> GESTURE_END. The
SCROLL_END event will get processed and Hide the non-resizable app,
which will trigger the backdrop to destroy. Then the GESTURE_END will
crash when it tries to do coordinate conversion.

To fix this, move coordinate conversion outside the loop as the results
should be the same.

[1] https://docs.google.com/document/d/1mQFx99xz5jVUHQk2jZTjHPgMjRQd4EUDfdLG0XKOlWI/edit

Test: added test
Bug: 1064618
Change-Id: I91c9e0955d01a651a779a800149a73f6c545a767
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2119632
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753769}