[heap] Fix registering HashTable backings

The backing table should only be registered when processing the
HashTable as otherwise the backing might be black but its fields might
not be processed if it gets dropped till the final atomic pause.

The registration will still be early enough to announce the slot to

Bug: chromium:875555
Change-Id: Iad97df11f79d95524839fd0377585bc68b2442ea
Reviewed-on: https://chromium-review.googlesource.com/1181130
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Keishi Hattori <keishi@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584678}
1 file changed