Move ShareableBitmap from WebKit down to WebCore. This will facilitate IPC serialization of WebCore::Image.
Pull request: https://github.com/WebKit/WebKit/pull/23722
Committed 273995@main (e1220dfe8a4e): <https://commits.webkit.org/273995@main> Reviewed commits have been landed. Closing PR #23722 and removing active labels.
<rdar://problem/122190394>