We should probably disable it by default and enable only For Android.
Needs the I2S to get approved

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/330caa67425b84037410512f8e032761eb6fc968

commit 330caa67425b84037410512f8e032761eb6fc968
Author: zqzhang <zqzhang@chromium.org>
Date: Wed Jan 18 15:31:15 2017

Enable MediaSession API on Android

Turn on the flag as we got approval on the intent to ship:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/xIpJ_303phw

BUG= 679303 

Review-Url: https://codereview.chromium.org/2639633002
Cr-Commit-Position: refs/heads/master@{#444366}

[modify] https://crrev.com/330caa67425b84037410512f8e032761eb6fc968/content/child/runtime_features.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/054f78b8996675673b5dab034b5b7f2698e4e9d5

commit 054f78b8996675673b5dab034b5b7f2698e4e9d5
Author: zqzhang <zqzhang@chromium.org>
Date: Thu Jan 19 17:56:53 2017

[Blink>Media] Remove experimental flag for MediaSession from RuntimeEnabledFeatures

As Media Session API is enabled by default on Android, and
disabled for other platforms, we need to mark it as "test" so it
won't be enabled when "experimental web platform features" is
enabled, while it's enabled in tests.

BUG= 679303 

Review-Url: https://codereview.chromium.org/2638413002
Cr-Commit-Position: refs/heads/master@{#444781}

[modify] https://crrev.com/054f78b8996675673b5dab034b5b7f2698e4e9d5/third_party/WebKit/Source/platform/RuntimeEnabledFeatures.in