Adds image label option to the context menus when a screen reader is enabled.

Also adds a bubble to confirm that a user wants the setting on. This copies
the behavior of the spelling suggestions service.

Context menu change is only added when the feature flag is enabled.

Bug: 905419
Change-Id: I171f21a9766d150b113e15a0f95238ed435fda5e
Reviewed-on: https://chromium-review.googlesource.com/c/1404156
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Katie Dektar <katie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622107}
22 files changed