Fix initial focus finding, ignore div nodes, and resolve aria-label name calculation.
Specifically, the divs inside of OOBE are now appropriately ignored, focus drills down into descendant focus nodes whenever a root web area gains focus, and we use aria-label for the name when there's no name property. For the last issue, this is just a temporary work around until the name property provides the finalized name.
TEST=run through OOBE, tab into a root web area, navigate and verify no divs get ChromeVox focus, and focus a text field (password field) on the login screen.
BUG=488693
Review URL: https://codereview.chromium.org/
1214233002
Cr-Commit-Position: refs/heads/master@{#338874}