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This change updates the HTTP request sent to the image annotation
server to request image descriptions as well as OCR, and updates
the service API to expose this data.

This change has been manually tested, but more unittests are to
come shortly in another CL.

Bug: 916420

Change-Id: Ifd919c376f117e25db93389ab4091c18c684e016
Reviewed-on: https://chromium-review.googlesource.com/c/1478652
Commit-Queue: Michael Martis <martis@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Sam McNally <sammc@chromium.org>
Reviewed-by: Martin Šrámek <msramek@chromium.org>
Reviewed-by: Andrew Moylan <amoylan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635423}