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[DevicePage] Make visibility assertions consistent

As khorimoto@ pointed out on crrev.com/c/2471900, each assertion (or
expectation) looking at `offsetHeight` directly is rather hard to read,
and doesn't make it clear what's actually being tested. This CL changes
all visibility assertions and expectations to use the same `isVisible`
method.

Bug: none
Test: check the tests pass, and fail if `hidden` is added to an element
Change-Id: I21dacaca741324c20b671748cea71e4c885b9d7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2484931Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Harry Cutts <hcutts@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818583}