Merge M56: "Add audio only watch time metrics."

Merge M56: "Add audio only watch time metrics."

Enables tracking of audio only "watch time" histograms. They are
the same as the audio+video ones except there is no requirement
on visibility or video size for recording watch time.

BUG=633743
TEST=new audio only unittests.

Review-Url: https://codereview.chromium.org/2498113004
Cr-Commit-Position: refs/heads/master@{#433102}
(cherry picked from commit 051d5b8e2973c0f253ade88303b467a043530d23)

Committed: https://chromium.googlesource.com/chromium/src/+/7c02dce64f7c158bee54e9f161bfffd71d45d2dc

[DaleCurtis, 2016-11-21 20:30:31]

Description was changed from

==========
Merge M56: "Add audio only watch time metrics."

Enables tracking of audio only "watch time" histograms. They are
the same as the audio+video ones except there is no requirement
on visibility or video size for recording watch time.

BUG=633743
TEST=new audio only unittests.

Review-Url: https://codereview.chromium.org/2498113004
Cr-Commit-Position: refs/heads/master@{#433102}
(cherry picked from commit 051d5b8e2973c0f253ade88303b467a043530d23)
==========

to

==========
Merge M56: "Add audio only watch time metrics."

Enables tracking of audio only "watch time" histograms. They are
the same as the audio+video ones except there is no requirement
on visibility or video size for recording watch time.

BUG=633743
TEST=new audio only unittests.

Review-Url: https://codereview.chromium.org/2498113004
Cr-Commit-Position: refs/heads/master@{#433102}
(cherry picked from commit 051d5b8e2973c0f253ade88303b467a043530d23)

Committed:
https://chromium.googlesource.com/chromium/src/+/7c02dce64f7c158bee54e9f161bf...
==========

[DaleCurtis, 2016-11-21 20:30:32]

Committed patchset #1 (id:1) manually as
7c02dce64f7c158bee54e9f161bfffd71d45d2dc.