I think it's reasonable to continue to use Ash.Wallpaper.ColorExtraction.Durations and Ash.Wallpaper.ColorExtraction.UserDelay as-is so long as we update the descriptions to note that it includes multiple color profiles.  In the description, be sure to state the version when the changes apply.

As for the Ash.Wallpaper.ColorExtractionResult we should probably replace it with something new.  As per https://chromium.googlesource.com/chromium/src.git/+/HEAD/tools/metrics/histograms/README.md#Multidimensional-Histograms I would lean towards an enum histogram that crosses the luma, saturation, and the result, which would yield an enum with 12 values.

moved from email discussion:

estade: the performance impact would be immeasurably small for calculating six colors in one time

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/31e3bc75709ac8c54da431bee36fcec5b4d432c5

commit 31e3bc75709ac8c54da431bee36fcec5b4d432c5
Author: warx <warx@chromium.org>
Date: Thu Jun 22 21:24:59 2017

Extracting more than one wallpaper prominent color

Changes:
This is basically a refactor of code to support more than one wallpaper
prominent color extracting.
(1) group LumaRange and SaturationRange to ColorProfile, and a vector
of color_profiles are passed as parameter for calculation.
(2) updated UMA for Ash.Wallpaper.ColorExtractionResult to enumeration.

R=xdai@chromium.org, bruthig@chromium.org, estade@chromium.org,
danakj@chromium.org, isherman@chromium.org, jamescook@chromium.org
BUG= 734761 
TEST=covered by tests

Review-Url: https://codereview.chromium.org/2943333003
Cr-Commit-Position: refs/heads/master@{#481667}

[modify] https://crrev.com/31e3bc75709ac8c54da431bee36fcec5b4d432c5/ash/shelf/shelf_background_animator.cc
[modify] https://crrev.com/31e3bc75709ac8c54da431bee36fcec5b4d432c5/ash/wallpaper/wallpaper_controller.cc
[modify] https://crrev.com/31e3bc75709ac8c54da431bee36fcec5b4d432c5/ash/wallpaper/wallpaper_controller.h
[modify] https://crrev.com/31e3bc75709ac8c54da431bee36fcec5b4d432c5/chrome/browser/chromeos/login/users/wallpaper/wallpaper_manager.cc
[modify] https://crrev.com/31e3bc75709ac8c54da431bee36fcec5b4d432c5/chrome/browser/chromeos/login/users/wallpaper/wallpaper_manager.h
[modify] https://crrev.com/31e3bc75709ac8c54da431bee36fcec5b4d432c5/components/wallpaper/BUILD.gn
[modify] https://crrev.com/31e3bc75709ac8c54da431bee36fcec5b4d432c5/components/wallpaper/wallpaper_color_calculator.cc
[modify] https://crrev.com/31e3bc75709ac8c54da431bee36fcec5b4d432c5/components/wallpaper/wallpaper_color_calculator.h
[modify] https://crrev.com/31e3bc75709ac8c54da431bee36fcec5b4d432c5/components/wallpaper/wallpaper_color_calculator_unittest.cc
[add] https://crrev.com/31e3bc75709ac8c54da431bee36fcec5b4d432c5/components/wallpaper/wallpaper_color_extraction_result.h
[modify] https://crrev.com/31e3bc75709ac8c54da431bee36fcec5b4d432c5/tools/metrics/histograms/enums.xml
[modify] https://crrev.com/31e3bc75709ac8c54da431bee36fcec5b4d432c5/tools/metrics/histograms/histograms.xml
[modify] https://crrev.com/31e3bc75709ac8c54da431bee36fcec5b4d432c5/ui/gfx/color_analysis.cc
[modify] https://crrev.com/31e3bc75709ac8c54da431bee36fcec5b4d432c5/ui/gfx/color_analysis.h
[modify] https://crrev.com/31e3bc75709ac8c54da431bee36fcec5b4d432c5/ui/gfx/color_analysis_unittest.cc