Ran quick audit using axe-core. Found a few minor issues, created CL: https://crrev.com/c/1343246

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3311037f2f5da733da840f53a65610b09fa32617

commit 3311037f2f5da733da840f53a65610b09fa32617
Author: Hector Carmona <hcarmona@chromium.org>
Date: Wed Nov 28 19:27:18 2018

Navi: Fix minor a11y issues found with audit

Bug:  882586 
Change-Id: Id18e17c72aa3646a52a074e536fafa5cf789d85c
Reviewed-on: https://chromium-review.googlesource.com/c/1343246
Reviewed-by: Scott Chen <scottchen@chromium.org>
Commit-Queue: Hector Carmona <hcarmona@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611818}
[modify] https://crrev.com/3311037f2f5da733da840f53a65610b09fa32617/chrome/browser/resources/welcome/onboarding_welcome/shared/onboarding_background.html