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- Allow downcast construction;
- Disallow conversion to T* directly;
- Disallow Get();

This CL concludes hardening as all uses left go through an
intermediate CTP handle which retains the object as long
as the thread does not terminate.

Bug: 1125495
Change-Id: I4ab837127745378a57513d11ffbdea3fd961b20b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2396140
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805306}