Add threading assertion to Identified::generateIdentifier() and fix bug found by the assertion.
Pull request: https://github.com/WebKit/WebKit/pull/16570
Committed 266798@main (0f4cde430d88): <https://commits.webkit.org/266798@main> Reviewed commits have been landed. Closing PR #16570 and removing active labels.
<rdar://problem/113722535>