Fix UAF in SyntheticPointerAction

https://crrev.com/c/4241725 fixed a UAF where a gesture action causes
the browser window to be closed and the gesture controller to be
destroyed. It did so by checking for the controller's destruction after
any click-type event dispatch and early returning to ensure we're not
touching any of the gesture's members after destroying it.

That CL missed one case in SyntheticPointerAction. While
ForwardInputEvents does check for destruction and return GESTURE_ABORT,
the call to ForwardTouchOrMouseInputEvents has some code after dispatch
that will read memory belonging to |this|. This CL adds an early return
in this case as well.

Bug: 1427918,1394736
Change-Id: I08bf3a42f0bcaa44b2a795021fb5f8e32b35a67a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4375537


Reviewed-by: Mustaq Ahmed <mustaq@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1123742}