With site isolation we often need to check if a remote page proxy exists for a given frame before sending IPC to the web page hosting the frame. We should have a helper function instead of duplicating this logic throughout WebPageProxy.
<rdar://problem/118368823>
Pull request: https://github.com/WebKit/WebKit/pull/20455
Committed 270723@main (7d27cdd3e980): <https://commits.webkit.org/270723@main> Reviewed commits have been landed. Closing PR #20455 and removing active labels.