The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/de3c6b4958e9d75c8e76c4a314de532846f94aa8

commit de3c6b4958e9d75c8e76c4a314de532846f94aa8
Author: Xiaohan Wang <xhwang@chromium.org>
Date: Tue Nov 21 19:08:26 2017

media: Report connection error UMA in MojoCdm

This will report a baseline event every time MojoCdm is created
(unless connection error already happens during connection time).
Then it will report an event if mojo connection error happens, which
typically means the CDM process crashed.

BUG= 785713 
TEST=Manually tested by inspecting about://histograms

Change-Id: I0569c98435dcadd871774ec22234feb5c703110b
Reviewed-on: https://chromium-review.googlesource.com/780047
Reviewed-by: Frank Liberato <liberato@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518324}
[modify] https://crrev.com/de3c6b4958e9d75c8e76c4a314de532846f94aa8/media/mojo/clients/mojo_cdm.cc
[modify] https://crrev.com/de3c6b4958e9d75c8e76c4a314de532846f94aa8/media/mojo/clients/mojo_decryptor.cc
[modify] https://crrev.com/de3c6b4958e9d75c8e76c4a314de532846f94aa8/media/mojo/clients/mojo_decryptor.h
[modify] https://crrev.com/de3c6b4958e9d75c8e76c4a314de532846f94aa8/tools/metrics/histograms/enums.xml
[modify] https://crrev.com/de3c6b4958e9d75c8e76c4a314de532846f94aa8/tools/metrics/histograms/histograms.xml