heap: Filter already marked weak handles

The write barrier will ensure that newly set values will be marked black
in any case and thus make the callback obsolete.

Bug: 843903
Change-Id: I058df9c4e21ba4a5045ea6890b5ee8d5b0e7fc27
Reviewed-on: https://chromium-review.googlesource.com/c/1372788
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615835}
1 file changed