Use base::apple::Owned[frameworktype] for ui::PlatformEvent

base::apple::Owned[frameworktype] is a new way to easily generate
owning wrapper classes for framework types. This CL switches
ui::PlatformEvent to use it, and does cleanup.

This unlocks the ability to convert ui/events/ to use ARC.

Bug: 1443009
Bug: 1433041
Bug: 1280317
Change-Id: Ib1ffd2f54aa1ba641eebbabe3ed4349b026650f3
Include-Ci-Only-Tests: true
Cq-Include-Trybots: luci.chromium.try:ios-blink-dbg-fyi
Cq-Include-Trybots: luci.chrome.try:mac-chrome
Validate-Test-Flakiness: skip
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4562796


Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
Auto-Submit: Avi Drissman <avi@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1149240}