[M131] Fix a crash on pdf download complete

The PdfPage may become a FrozenNativePage while downloading.
Need to check before cast to PdfPage.

(cherry picked from commit a859c025)

Low-Coverage-Reason: TRIVIAL_CHANGE
Bug: 377305897
Change-Id: I4eefe7de95e344a07ede7c94babe3352d626f229
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5985554


Commit-Queue: Shu Yang <shuyng@google.com>
Reviewed-by: Min Qin <qinmin@chromium.org>
Cr-Original-Commit-Position: refs/heads/main@{#1377765}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5989896
Cr-Commit-Position: refs/branch-heads/6778@{#1726}
Cr-Branched-From: b21671ca-refs/heads/main@{#1368529}

chrome/android/java/src/org/chromium/chrome/browser/download/DownloadController.java

@@ -58,11 +58,14 @@ public class DownloadController {
         if (nativePage == null || !nativePage.isPdf()) {
             return;
         }
-        assert nativePage instanceof PdfPage;
-        ((PdfPage) nativePage)
-                .onDownloadComplete(
-                        downloadInfo.getFileName(), downloadInfo.getFilePath(), isDownloadSafe);
-        tab.updateTitle();
+        // The PdfPage may become a FrozenNativePage while downloading.
+        // Need to check before cast to PdfPage.
+        if (nativePage instanceof PdfPage) {
+            ((PdfPage) nativePage)
+                    .onDownloadComplete(
+                            downloadInfo.getFileName(), downloadInfo.getFilePath(), isDownloadSafe);
+            tab.updateTitle();
+        }
     }
 
     /**