[UnifiedPDF] PDFDocumentLayout::indexForPage should use equivalent PD…

…FKit API.

https://bugs.webkit.org/show_bug.cgi?id=282219
rdar://138139244

Reviewed by Aditya Keerthi.

After some discussion it turns out that we should not have our own indexForPage
implementation on top of other PDFKit APIs and instead we should just unconditionally
adopt the equivalent API provided by PDFKit. This was causing some issues with find in
certain PDFs and using [PDFDocument indexForPage] resolves these issues.

* Source/WebKit/WebProcess/Plugins/PDF/UnifiedPDF/PDFDocumentLayout.mm:
(WebKit::PDFDocumentLayout::indexForPage const):

Canonical link: https://commits.webkit.org/285823@main

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

@@ -81,11 +81,11 @@
 
 auto PDFDocumentLayout::indexForPage(RetainPtr<PDFPage> page) const -> std::optional<PageIndex>
 {
-    for (PageIndex pageIndex = 0; pageIndex < [m_pdfDocument pageCount]; ++pageIndex) {
-        if (page == [m_pdfDocument pageAtIndex:pageIndex])
-            return pageIndex;
-    }
-    return std::nullopt;
+
+    auto pageIndex = [m_pdfDocument indexForPage:page.get()];
+    if (pageIndex == NSNotFound)
+        return { };
+    return pageIndex;
 }
 
 PDFDocumentLayout::PageIndex PDFDocumentLayout::nearestPageIndexForDocumentPoint(FloatPoint documentSpacePoint, const std::optional<PDFLayoutRow>& visibleRow) const