Detect root layer in throttled iframe during compositing input update

Bug: 1038959
Change-Id: I5151afda08af17758ddbbb0ff1ac2675746903b1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2698618


Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/master@{#854824}