Fix PrintPreviewScalingSettingsTest.SetScaling to pass blink's A11y checks.

Stricter a11y checks that have been added within Blink itself were
triggering during these test's execution. Specifically the following
error

"Blocked aria-hidden on an element because its descendant retained
focus..."

This was happening because the test code was programmatically changing
the value of the scaling settings <select> control, which in turn
controls the visibility of a numerical <input> which was focused. The
violation was only happening because the test did not closely simulate
how a user would interact with the <select>, where the <select> would
have to be focused first.

This will unblock turning such violations to DCHECK()s so that they are
actually caught by the linux-blink-web-tests-force-accessibility-rel
bot.

Bug: None
Change-Id: Ied96be023f7d37d0f244841611be7dfe89752654
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6333657
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1429212}