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This CL is the third 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. We have also hidden the
entire feature behind a feature flag, so this CL is not visible
to an average user.

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This CL adds the following:

- Adds helper text/hint for unlabeled images
- Adds returned image description to node's inner text
- Associated strings etc for above

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Binary-Size: Size increase is unavoidable, it comes from adding necessary content strings and descriptions for translations.
AX-Relnotes: Adds feature to enable generating alt image descriptions, currently behind a feature flag.
Bug: 1057169
Change-Id: I6becb973045e127681136a1fe0b7171d5cbddca8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2317712Reviewed-by: danakj <danakj@chromium.org>
Reviewed-by: Mark Schillaci <mschillaci@google.com>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Mark Schillaci <mschillaci@google.com>
Cr-Commit-Position: refs/heads/master@{#821324}