Move some checks to PDFiumEngine::AppendPage()

Make PDFiumEngine::AppendPage() check its input parameters, instead of
the caller. Change these DCHECKs into CHECKs, per modern coding
guidance, since they are cheap to run. This also helps to catch a rare
crash where it looks as though the impossible has happened.

Bug: 1488743
Change-Id: Ia491b20b771beebc0cd968908873228793e86698
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4903239
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Nigi <nigi@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1204952}

pdf/pdf_view_web_plugin.cc

@@ -2307,9 +2307,7 @@ void PdfViewWebPlugin::PreviewDocumentLoadComplete() {
   preview_document_load_state_ = DocumentLoadState::kComplete;
 
   int dest_page_index = preview_pages_info_.front().dest_page_index;
-  DCHECK_GT(dest_page_index, 0);
   preview_pages_info_.pop();
-  DCHECK(preview_engine_);
   engine_->AppendPage(preview_engine_.get(), dest_page_index);
 
   ++print_preview_loaded_page_count_;

pdf/pdfium/pdfium_engine.cc

@@ -759,6 +759,9 @@ std::unique_ptr<URLLoaderWrapper> PDFiumEngine::CreateURLLoader() {
 }
 
 void PDFiumEngine::AppendPage(PDFEngine* engine, int index) {
+  CHECK(engine);
+  CHECK(PageIndexInBounds(index));
+
   // Unload and delete the blank page before appending.
   pages_[index]->Unload();
   pages_[index]->set_calculated_links(false);