+1. I have a TODO for this in AudioFocusManager :)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/238868bfde91027175160a06ce46382124acc1d9

commit 238868bfde91027175160a06ce46382124acc1d9
Author: zqzhang <zqzhang@chromium.org>
Date: Wed Aug 31 11:09:21 2016

Let MediaSession store duck state instead of volume multiplier

BUG= 642361 

Review-Url: https://codereview.chromium.org/2291163002
Cr-Commit-Position: refs/heads/master@{#415608}

[modify] https://crrev.com/238868bfde91027175160a06ce46382124acc1d9/content/browser/media/session/audio_focus_manager.cc
[modify] https://crrev.com/238868bfde91027175160a06ce46382124acc1d9/content/browser/media/session/audio_focus_manager_unittest.cc
[modify] https://crrev.com/238868bfde91027175160a06ce46382124acc1d9/content/browser/media/session/media_session.cc
[modify] https://crrev.com/238868bfde91027175160a06ce46382124acc1d9/content/browser/media/session/media_session.h
[modify] https://crrev.com/238868bfde91027175160a06ce46382124acc1d9/content/browser/media/session/media_session_browsertest.cc
[modify] https://crrev.com/238868bfde91027175160a06ce46382124acc1d9/content/browser/media/session/media_session_delegate_android.cc
[modify] https://crrev.com/238868bfde91027175160a06ce46382124acc1d9/content/browser/media/session/media_session_delegate_android.h
[modify] https://crrev.com/238868bfde91027175160a06ce46382124acc1d9/content/public/android/java/src/org/chromium/content/browser/MediaSessionDelegate.java