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This CL is the seventh part of an effort to port the existing Desktop
feature to Android. This feature enables a user to send an image to
Google to process to generate a descriptive alt text if a website
does not provide one.

Design Doc: go/2020-q1-android-image-descriptions
Slide Deck: go/clank-imageDescriptions
Launch Bug: 1057168

Original Desktop Design Doc for reference:
go/chrome-a11y-annotations-design

This is a conservative approach, we use a separate set of profile
Prefs for this feature rather than syncing with the Desktop Prefs.
This can be updated in time as needed.

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This CL adds the following:

- Syncs Regular and Incognito behavior as per new reqs
- Corrects unit tests for above
- Adds an AXMode observer to watch for screen readers being enabled

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AX-Relnotes: Android accessibility image descriptions behavior in incognito tabs will now match the regular tabs behavior for that profile.
Bug: 1057169
Change-Id: I27b6cb1c3da4cae441a6519815f7835049827452
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2585587Reviewed-by: Theresa  <twellington@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Mark Schillaci <mschillaci@google.com>
Commit-Queue: Mark Schillaci <mschillaci@google.com>
Cr-Commit-Position: refs/heads/master@{#836174}