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CTWP is a weak ref used on a different thread than the object has been
original been allocated on. Using the object directly is inherently
unsafe as garbage collection only consider the stack of the thread the
heap is attached to. For that reason, working on an object must
require a strong reference.

Note that this does not rule out issues, as the originating thread may
terminate in which case even the strong CrossThreadPersistent does not
keep the object alive.

Nonetheless, the only valid use case of a CTWP should be weakly
pointing to an object. The CL introduces a Lock() call, similar to
std::weak_ptr::Lock() with the same semantics: The reference (CTP)
needs to be check against cleared'ness before the object can be used.

Blink: Most use cases rely on external synchronizations when accessing
a CTWP, so this change merely hardens the API but likely does not fix
any real issues.

Bug: 1125495
Change-Id: Ib859df5e9c5f418aa36797c4bd9a444de8c82b15
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2391231
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804864}