* Previous undesired outcome: Script window can be null and dereference
  nullptr results in crash.
* Root cause: We suspect script from iframe could have their dom window
  detached by the time animation_frame_timing_monitor doing 3p script
  detection after BeginMainFrame.
* This CL address it by: use local root frame's window instead to
  retrieve 3p script detector; also added a check to early exit just in
  case.

(cherry picked from commit 2931c641ca38226a1881fd6e52cbc328a9225cab)

Bug: 336375355, 336503423, 336517147
Change-Id: I4e21e0b133fae2e52f50b42c7f42d21848e17eb3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5479330
Reviewed-by: Ian Clelland <iclelland@chromium.org>
Reviewed-by: Noam Rosenthal <nrosenthal@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Aoyuan Zuo <zuoaoyuan@chromium.org>
Cr-Original-Commit-Position: refs/heads/main@{#1292012}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5484155
Reviewed-by: Aoyuan Zuo <zuoaoyuan@chromium.org>
Owners-Override: Krishna Govind <govind@chromium.org>
Reviewed-by: Krishna Govind <govind@chromium.org>
Cr-Commit-Position: refs/branch-heads/6437@{#5}
Cr-Branched-From: 4af79fa7d07888d7786fde7c342af0085721c833-refs/heads/main@{#1291458}