[UnifiedPDF] UnifiedPDFPlugin::scaleFactor() always reports a scale…

… factor of 1

https://bugs.webkit.org/show_bug.cgi?id=266035
rdar://119344324

Reviewed by Tim Horton and Simon Fraser.

Let's make it report `m_scaleFactor` instead, since it isn't guaranteed
that the scale factor will be 1.

* Source/WebKit/WebProcess/Plugins/PDF/UnifiedPDF/UnifiedPDFPlugin.mm:
(WebKit::UnifiedPDFPlugin::scaleFactor):

Canonical link: https://commits.webkit.org/271707@main

Source/WebKit/WebProcess/Plugins/PDF/UnifiedPDF/UnifiedPDFPlugin.mm

@@ -297,7 +297,7 @@
 
 CGFloat UnifiedPDFPlugin::scaleFactor() const
 {
-    return 1;
+    return m_scaleFactor;
 }
 
 float UnifiedPDFPlugin::deviceScaleFactor() const