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Tracing a Member or a WeakMember first checks whether the member is
deleted. If the check passes, the member is traced. The check and
the trace both use atomic loads, but not the same load. This can
result in time-of-check vs time-of-use data races. This CL merges
both loads to a single load to avoid such issues.

Furthermore, members in hash tables now might be deleted while the
relevant entry is deleted. Since the is deleted check is performed
on a copy, member types need to bail out on deleted values (used to
be a DCHECK before).

Bug: 986235
Change-Id: I4307361af94081b62d159af67cdbde5666378550
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2012285
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735356}