The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5a2ee1339095755400f7249f6add4e97d29717c6

commit 5a2ee1339095755400f7249f6add4e97d29717c6
Author: eustas <eustas@chromium.org>
Date: Tue May 24 09:15:24 2016

TL;DR: adding "gzip-header-detected" and "brotli-error-code" UMA's

BrotliFilter.Status UMA's show that about 0.5% of requests finish with "In Progress" status and 0.05% with "Error" status.

The same data sliced by platform:

            | Windows | Android |
------------+---------+---------+
In progress |   0.57% |   0.29% |
Error       |   0.00% |   0.15% |
------------+---------+---------+
          Σ |   0.57% |   0.44% |

The sum for Windows and Android looks similar.

This makes me suspicious that it both results may have the same reason:
actual content has invalid (non-brotli) encoding.

To check this I plan to add additional UMA's: one will report if gzip header is detected, the other one will report brotli error code that will allow to understand at what point "corrupted" input is detected.

BUG=613113

Review-Url: https://codereview.chromium.org/1995513002
Cr-Commit-Position: refs/heads/master@{#395552}

[modify] https://crrev.com/5a2ee1339095755400f7249f6add4e97d29717c6/net/filter/brotli_filter.cc
[modify] https://crrev.com/5a2ee1339095755400f7249f6add4e97d29717c6/tools/metrics/histograms/histograms.xml

Moving this nonessential bug to the next milestone.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

This issue is Pri-1 but has already been moved once. Lowering the priority and moving to the next milestone.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

            | Windows | Android |  OS X  |
------------+---------+---------+--------|
In progress |  0.069% |  0.193% | 0.067% |
Error       |  0.002% |  0.000% | 0.001% |
------------+---------+---------+--------|
          Σ |  0.071% |  0.193% | 0.067% |

Error rate on Android improved a lot. Overall "not-Done" rate is 10x lower nowadays. It seems that content producers / proxies has fixed their pipelines.