Harden ToolbarSnapshotState to check for nulls for visual hint checks.

This also moves the assertion to the constructor to make it easier to see where the errors are being triggered.

BUG=1383684

Change-Id: I7cf0e4ff2c45791c622e519bd461523d824585e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4027036


Reviewed-by: Sky Malice <skym@chromium.org>
Commit-Queue: Ted Choc <tedchoc@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1071924}