The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/54d60411fa5e3e4077f9369ce153d8701b72f870

commit 54d60411fa5e3e4077f9369ce153d8701b72f870
Author: Wenzhao Zang <wzang@chromium.org>
Date: Tue Feb 13 19:12:00 2018

cros: Implement 'My Photos' section in new wallpaper picker (Part I)

The process of generating and showing the thumbnails for local images can be
divided into:

1) The wallpaper picker gets the names of all images files under the Downloads
   directory (implemented in this CL).

2) The picker requests the thumbnails to be generated for each file name.
   (We want to separate the two steps, since generating thumbnails can be an
   expensive operation. The wallpaper picker doesn't have to wait until all
   the thumbnails to be created before showing anything.)

3) When user clicks on a thumbnail, set the image corresponding to it as the
   custom wallpaper (This part will reuse the existing code.)

Bug:  809793 
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I0786453259d5dc1dae71bcf8c6d9624f31d91a01
Reviewed-on: https://chromium-review.googlesource.com/905809
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Toni Barzic <tbarzic@chromium.org>
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536410}
[modify] https://crrev.com/54d60411fa5e3e4077f9369ce153d8701b72f870/chrome/browser/chromeos/extensions/wallpaper_private_api.cc
[modify] https://crrev.com/54d60411fa5e3e4077f9369ce153d8701b72f870/chrome/browser/chromeos/extensions/wallpaper_private_api.h
[modify] https://crrev.com/54d60411fa5e3e4077f9369ce153d8701b72f870/chrome/browser/resources/chromeos/wallpaper_manager/js/wallpaper_manager.js
[modify] https://crrev.com/54d60411fa5e3e4077f9369ce153d8701b72f870/chrome/common/extensions/api/wallpaper_private.json
[modify] https://crrev.com/54d60411fa5e3e4077f9369ce153d8701b72f870/extensions/browser/extension_function_histogram_value.h
[modify] https://crrev.com/54d60411fa5e3e4077f9369ce153d8701b72f870/tools/metrics/histograms/enums.xml

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a3aeee0c9a0d3ab1f0a3c1dfddb6ea68388e9fd0

commit a3aeee0c9a0d3ab1f0a3c1dfddb6ea68388e9fd0
Author: Wenzhao Zang <wzang@chromium.org>
Date: Thu Feb 15 05:55:05 2018

cros: Implement 'My Photos' section in new wallpaper picker (Part II)

The old wallpaper picker must generate and save a thumbnail for each
custom wallpaper, so that they can persist even after the original
image file is deleted. This is not needed for the new picker:
the thumbnails under 'My Photos' section are real-time representation
of the files under Downloads. We do not want to cache or save them
since the files may change all the time.

Therefore, to simplify the logic, we only need to read the image data,
and then use CSS to resize the image to display as the thumbnail
(the same with backdrop wallpapers for consistency).

The new wallpaper picker doesn't display the thumbnail of the history
custom wallpaper like the old picker. So basically the 'generate and
save thumbnail' code path can be deprecated in the future.

Bug:  809793 
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Ibf00bec636a52081c77cd7cf1b94e934fd97b11c
Reviewed-on: https://chromium-review.googlesource.com/912282
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Toni Barzic <tbarzic@chromium.org>
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#536950}
[modify] https://crrev.com/a3aeee0c9a0d3ab1f0a3c1dfddb6ea68388e9fd0/chrome/browser/chromeos/extensions/wallpaper_private_api.cc
[modify] https://crrev.com/a3aeee0c9a0d3ab1f0a3c1dfddb6ea68388e9fd0/chrome/browser/chromeos/extensions/wallpaper_private_api.h
[modify] https://crrev.com/a3aeee0c9a0d3ab1f0a3c1dfddb6ea68388e9fd0/chrome/browser/resources/chromeos/wallpaper_manager/js/util.js
[modify] https://crrev.com/a3aeee0c9a0d3ab1f0a3c1dfddb6ea68388e9fd0/chrome/browser/resources/chromeos/wallpaper_manager/js/wallpaper_images_grid.js
[modify] https://crrev.com/a3aeee0c9a0d3ab1f0a3c1dfddb6ea68388e9fd0/chrome/browser/resources/chromeos/wallpaper_manager/js/wallpaper_manager.js
[modify] https://crrev.com/a3aeee0c9a0d3ab1f0a3c1dfddb6ea68388e9fd0/chrome/common/extensions/api/wallpaper_private.json
[modify] https://crrev.com/a3aeee0c9a0d3ab1f0a3c1dfddb6ea68388e9fd0/extensions/browser/extension_function_histogram_value.h
[modify] https://crrev.com/a3aeee0c9a0d3ab1f0a3c1dfddb6ea68388e9fd0/tools/metrics/histograms/enums.xml