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There may be existing pointers to the memory block that are being actively
accessed while the zeroing of memory is taking place. While this change
doesn't fix that, it does make it possible for those other threads to recognize
that memory is changing under it and be able to react appropriately.

Also, restore missing kTypeIdAny that was somehow lost.

BUG=620813

Review-Url: https://codereview.chromium.org/2709113003
Cr-Commit-Position: refs/heads/master@{#456121}