In addition of rappor metrics, I think we could use histograms that record usage of the three properties of the MediaSession interface. We can easily do that with [Measure] in IDl.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ba54980e209fedd167b84a09b7a075f4c7d5bbf5

commit ba54980e209fedd167b84a09b7a075f4c7d5bbf5
Author: zqzhang <zqzhang@chromium.org>
Date: Fri Jan 27 23:57:09 2017

Add rappor metrics to track Media Session API usage

BUG= 685615 

Review-Url: https://codereview.chromium.org/2651203005
Cr-Commit-Position: refs/heads/master@{#446839}

[modify] https://crrev.com/ba54980e209fedd167b84a09b7a075f4c7d5bbf5/third_party/WebKit/Source/modules/mediasession/MediaSession.cpp
[modify] https://crrev.com/ba54980e209fedd167b84a09b7a075f4c7d5bbf5/tools/metrics/rappor/rappor.xml

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/45e7756a8b104e4adfa2b4aead538dcea259e695

commit 45e7756a8b104e4adfa2b4aead538dcea259e695
Author: zqzhang <zqzhang@chromium.org>
Date: Mon Jan 30 13:08:37 2017

[Blink>Media] Record MediaSession API interface usage metrics

BUG= 685615 

Review-Url: https://codereview.chromium.org/2660583003
Cr-Commit-Position: refs/heads/master@{#446975}

[modify] https://crrev.com/45e7756a8b104e4adfa2b4aead538dcea259e695/third_party/WebKit/Source/core/frame/UseCounter.h
[modify] https://crrev.com/45e7756a8b104e4adfa2b4aead538dcea259e695/third_party/WebKit/Source/modules/mediasession/MediaSession.idl
[modify] https://crrev.com/45e7756a8b104e4adfa2b4aead538dcea259e695/tools/metrics/histograms/histograms.xml

We need to merge the two CLs into M57

Your change meets the bar and is auto-approved for M57. Please go ahead and merge the CL to branch 2987 manually. Please contact milestone owner if you have questions.
Owners: amineer@(clank), cmasso@(bling), ketakid@(cros), govind@(desktop)

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

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/4bb23bb916db20688d6bfc5139ce883128cbdf1b

commit 4bb23bb916db20688d6bfc5139ce883128cbdf1b
Author: Zhiqiang Zhang <zqzhang@google.com>
Date: Mon Jan 30 20:28:58 2017

Add rappor metrics to track Media Session API usage

BUG= 685615 

Review-Url: https://codereview.chromium.org/2651203005
Cr-Commit-Position: refs/heads/master@{#446839}
(cherry picked from commit ba54980e209fedd167b84a09b7a075f4c7d5bbf5)

Review-Url: https://codereview.chromium.org/2665733003 .
Cr-Commit-Position: refs/branch-heads/2987@{#180}
Cr-Branched-From: ad51088c0e8776e8dcd963dbe752c4035ba6dab6-refs/heads/master@{#444943}

[modify] https://crrev.com/4bb23bb916db20688d6bfc5139ce883128cbdf1b/third_party/WebKit/Source/modules/mediasession/MediaSession.cpp
[modify] https://crrev.com/4bb23bb916db20688d6bfc5139ce883128cbdf1b/tools/metrics/rappor/rappor.xml

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ac8fe85894c723ffe710af1e4d47a67db69ab841

commit ac8fe85894c723ffe710af1e4d47a67db69ab841
Author: Zhiqiang Zhang <zqzhang@google.com>
Date: Mon Jan 30 20:37:15 2017

[Blink>Media] Record MediaSession API interface usage metrics

BUG= 685615 

Review-Url: https://codereview.chromium.org/2660583003
Cr-Commit-Position: refs/heads/master@{#446975}
(cherry picked from commit 45e7756a8b104e4adfa2b4aead538dcea259e695)

Review-Url: https://codereview.chromium.org/2660793003 .
Cr-Commit-Position: refs/branch-heads/2987@{#182}
Cr-Branched-From: ad51088c0e8776e8dcd963dbe752c4035ba6dab6-refs/heads/master@{#444943}

[modify] https://crrev.com/ac8fe85894c723ffe710af1e4d47a67db69ab841/third_party/WebKit/Source/core/frame/UseCounter.h
[modify] https://crrev.com/ac8fe85894c723ffe710af1e4d47a67db69ab841/third_party/WebKit/Source/modules/mediasession/MediaSession.idl
[modify] https://crrev.com/ac8fe85894c723ffe710af1e4d47a67db69ab841/tools/metrics/histograms/histograms.xml