In https://bugs.webkit.org/show_bug.cgi?id=250847, Yijia Huang found a bug in Bitmap::concurrentTestAndClear() where it's returning the inverse of its expected result. Checking for all uses of concurrentTestAndClear(), we find that one result of IsoCellSet::remove() is also similarly incorrect. This patch fixes these errors, and also adds comments to document the intended return values of some relevant functions, as well as the reasoning behind how some of the return values are computed.
<rdar://problem/104590361>
Pull request: https://github.com/WebKit/WebKit/pull/9020
Committed 259287@main (3589dd6fe569): <https://commits.webkit.org/259287@main> Reviewed commits have been landed. Closing PR #9020 and removing active labels.