ProducerSharedCARingBuffer::allocate should be more robust in computing shared memory buffer size
<rdar://102920372>
Pull request: https://github.com/WebKit/WebKit/pull/13137
Committed 263407@main (18738469adb2): <https://commits.webkit.org/263407@main> Reviewed commits have been landed. Closing PR #13137 and removing active labels.