WKWebViewIOS already exposes this as a property with the getter `_isDisplayingPDF`. Let's bring parity to WKWebViewMac.
<rdar://problem/127442624>
Pull request: https://github.com/WebKit/WebKit/pull/28052
Committed 278324@main (dc1ebf3e8190): <https://commits.webkit.org/278324@main> Reviewed commits have been landed. Closing PR #28052 and removing active labels.