sadrul@, do you mind triaging this one?

Quick snapshot of the data on stable:

Windows:
0    4.40%    4.40%
1   95.31%   99.71%
2    0.16%   99.87%

Android:
0    2.92%    2.92%
1   29.37%   32.29%
2   33.78%   66.07%
3   27.29%   93.36%
4    6.24%   99.59%

The scale goes up to 10,000 [1].

So I guess the question is: should we remove the UMA? It looks like in over 99% of the cases, the scale factor is <= 4, which make sense, considering the default scale factor on the platforms. We can measure (and I suspect we already are) how many users are predominantly using devices of scale-factor=2,3,4 etc. Does this UMA tell us anything more that is interesting/useful?

[1] https://cs.chromium.org/chromium/src/cc/layers/picture_layer_impl.cc?l=1525

I've just landed
https://chromium-review.googlesource.com/c/chromium/src/+/905148

Which adds AddScaled(), AddKilo(), and AddKiB() methods in the base histogram class to increment the count by a scaled amount.  Precision is lost but accuracy is maintained.

You can't just change the Add* method used, though; the name of the histogram needs to be changed (add a "K" suffix?) to avoid confusion.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5301288489950bef3e6d0ce8156c777735bc34fa

commit 5301288489950bef3e6d0ce8156c777735bc34fa
Author: Sadrul Habib Chowdhury <sadrul@chromium.org>
Date: Sat Feb 10 06:27:51 2018

cc: Deprecate "Renderer4.IdealContentsScale" UMA.

The ideal content scale closely matches the default device scale factor,
which is retrieved in the uma results (as |primary_screen_scale_factor|
in system profile [1]). So deprecate Renderer4.IdealContentsScale.

[1] https://cs.chromium.org/chromium/src/third_party/metrics_proto/system_profile.proto?l=144

BUG= 809661 

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel
Change-Id: I0499f3973c794e3adf3741667259682f7d2346fe
Reviewed-on: https://chromium-review.googlesource.com/907496
Reviewed-by: vmpstr <vmpstr@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#535964}
[modify] https://crrev.com/5301288489950bef3e6d0ce8156c777735bc34fa/cc/layers/picture_layer_impl.cc
[modify] https://crrev.com/5301288489950bef3e6d0ce8156c777735bc34fa/tools/metrics/histograms/histograms.xml