Model validation support

Introduces a ModelValidator class and adds a command-line switch to
validate model using ModelValidator. No code is expected to run when the
command-line switch is not specified.

Bug: 1219668
Change-Id: I12f93956b4657bb9997b772a797f1e37d2e99a03
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2999073


Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Reviewed-by: Sophie Chang <sophiechang@chromium.org>
Commit-Queue: rajendrant <rajendrant@chromium.org>
Cr-Commit-Position: refs/heads/master@{#908596}