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}