[html-in-canvas] Prevent composited layers for inlines

We have code to prevent compositing reasons for canvas descendants,
other than direct children of the canvas. This code had an incorrect
check for LayoutBox, and was not run for inlines, resulting in
compositing.

Fixed: 493957490
Change-Id: Ic1eb62eb0e2965f358d5b356b395a1e90566aa24
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7688577
Commit-Queue: Stefan Zager <szager@chromium.org>
Auto-Submit: Philip Rogers <pdr@chromium.org>
Reviewed-by: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1603086}