Avoid setting selections in ChromeVox when not needed

Blink can be unstable when setting selections over non-text units (see bug).

R=dmazzoni@chromium.org

Fixed: 1104423
AX-Relnotes: fixes a crash when navigating in the Clear Browser Data settings page
Change-Id: I0eec2db11b958964dae92762f20707c3e6e18bb8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2315250


Commit-Queue: David Tseng <dtseng@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791578}