Adds initial scaffolding for image labeling feature

1. Adds another accessibility mode that turns the image labeling
feature on or off.

2. Adds UI under chrome://accessibility for controlling this mode either globally or on a per page basis.

3. Adds a new string attribute for the image label.

(Originally developed in this changelist: crrev.com/c/1460356)

TBR=tsepez@chromium.org

Bug: 905419
Change-Id: Iff77bf30316ff845256df7391e446aaae6c33cab
Reviewed-on: https://chromium-review.googlesource.com/c/1466025
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631308}
7 files changed