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The Blink.DecodedImage.JpegDensity.100px, 400px and 1000px metrics count
the number of images per image bpp (bits per pixel) in the three
different categories of image size (decodec image >= 1000px, >= 400px and
>= 100px), requiring three different metrics to cover the spectrum.

A more direct approach introduced by this commit is to track the number
of KiB per image density, since this will allow us to focus on the image
compression density ranges (measured in bpp) that matter the most for
the user since they are the compression density ranges that take the
most network bandwidth. This also eliminates the need to have three
metrics and simplifies the analysis.

The old Blink.DecodedImage.JpegDensity.100px, 400px and 1000px are
marked as deprecated after the next release by this commit, allowing to
have both old and new metrics overlap for one full release (M78).

Bug: None
Test: Added unittests.
Change-Id: I6ead60fa9da3bc1b6080b12a6044c3ff9c8ce50e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1742160
Commit-Queue: Philip Rogers <pdr@chromium.org>
Reviewed-by: Primiano Tucci <primiano@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#690390}