Merge of "Add metrics regarding concurrent audible tabs in Chromium."

Merge of "Add metrics regarding concurrent audible tabs in Chromium."

These metrics are keeping track of:
- whether there is another audible tab when a tab becomes audible;
- the maximum number of concurrent audible tab in a session;
- how long there are 2 or more audible tabs at the same time.

It is also recording when a tab gain or loses audible status.

BUG=578049
Review URL: https://codereview.chromium.org/1591453005

Cr-Commit-Position: refs/heads/master@{#370435}
(cherry picked from commit 44e4ef42ba9104b06ae212042452dfc81d89032b)

Committed: https://chromium.googlesource.com/chromium/src/+/1f83af1978b886e6dbe728bd793074c23293d55d

[mlamouri (slow - plz ping), 2016-01-21 18:15:13]

Description was changed from

==========
Merge of "Add metrics regarding concurrent audible tabs in Chromium."

These metrics are keeping track of:
- whether there is another audible tab when a tab becomes audible;
- the maximum number of concurrent audible tab in a session;
- how long there are 2 or more audible tabs at the same time.

It is also recording when a tab gain or loses audible status.

BUG=578049

Review URL: https://codereview.chromium.org/1591453005

Cr-Commit-Position: refs/heads/master@{#370435}
(cherry picked from commit 44e4ef42ba9104b06ae212042452dfc81d89032b)
==========

to

==========
Merge of "Add metrics regarding concurrent audible tabs in Chromium."

These metrics are keeping track of:
- whether there is another audible tab when a tab becomes audible;
- the maximum number of concurrent audible tab in a session;
- how long there are 2 or more audible tabs at the same time.

It is also recording when a tab gain or loses audible status.

BUG=578049

Review URL: https://codereview.chromium.org/1591453005

Cr-Commit-Position: refs/heads/master@{#370435}
(cherry picked from commit 44e4ef42ba9104b06ae212042452dfc81d89032b)

Committed:
https://chromium.googlesource.com/chromium/src/+/1f83af1978b886e6dbe728bd7930...
==========

[mlamouri (slow - plz ping), 2016-01-21 18:15:15]

Committed patchset #1 (id:1) manually as
1f83af1978b886e6dbe728bd793074c23293d55d.