The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/20cd2afe8e146f71fcd71e61d01e773fe8865510

commit 20cd2afe8e146f71fcd71e61d01e773fe8865510
Author: Xiaohan Wang <xhwang@chromium.org>
Date: Thu Oct 19 23:58:58 2017

media: Add UMA for CDM creation result

This UMA will report whether CDM instance creation succeeded or not. As
usually, we report it for Clear Key, Widevine and other (unknown) key
systems.

BUG= 700147 
TEST=Manually checked about://histograms

Change-Id: I0925d07c7a61da68ba43d2d170931617a4452f07
Reviewed-on: https://chromium-review.googlesource.com/727180
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Xiaohan Wang <xhwang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510258}
[modify] https://crrev.com/20cd2afe8e146f71fcd71e61d01e773fe8865510/media/blink/cdm_session_adapter.cc
[modify] https://crrev.com/20cd2afe8e146f71fcd71e61d01e773fe8865510/tools/metrics/histograms/histograms.xml