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.

This was committed as https://crrev.com/c/4562796 but reverted
as a broken LSan bot showed that the original change was
faulty. On Mac/Win/iOS the original platform event should be
copied when a ui::Event is copied, but on other platforms
the original platform event must not be copied. See the
diff from 1..3 to see the change from the last commit.

Bug: 1443009
Bug: 1433041
Bug: 1280317
Change-Id: I122dc2b9cb7b867092618d8691e4456e3e4ebb06
Include-Ci-Only-Tests: true
Cq-Include-Trybots: luci.chromium.try:ios-blink-dbg-fyi
Cq-Include-Trybots: luci.chrome.try:mac-chrome
Cq-Include-Trybots: luci.chromium.try:linux-lacros-asan-lsan-rel
Validate-Test-Flakiness: skip
Low-Coverage-Reason: events_stub is intentionally a fallback and not tested.
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4568966
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1149970}