[Video] Fix keyframe distance average calculations.

[Video] Fix keyframe distance average calculations.

Found a few problems with the way the keyframe distance is calculated and
passed from the decoder to WMPI:
1. as soon as the video decoder is stopped we reset the average - meaning we
will stop considering optimizing playback for the video
2. we have to wait for more than 4 keyframes to start optimizing video
playback, this can take seconds
3. audio decoder resets the average to 0 in pipeline's shared state

BUG=688618
TEST=Manual

Review-Url: https://codereview.chromium.org/2643103002
Cr-Commit-Position: refs/heads/master@{#444955}
(cherry picked from commit 8127a839bf18e618aaf7a8cad95440c6cd3f9f76)

Review-Url: https://codereview.chromium.org/2701513002 .
Cr-Commit-Position: refs/branch-heads/2987@{#526}
Cr-Branched-From: ad51088c0e8776e8dcd963dbe752c4035ba6dab6-refs/heads/master@{#444943}
Committed: https://chromium.googlesource.com/chromium/src/+/7837cbd3bb82ec56e15ca5f041ad59bf7d88a85d

[whywhat, 2017-02-15 19:32:25]

Description was changed from

==========
[Video] Fix keyframe distance average calculations.

Found a few problems with the way the keyframe distance is calculated and
passed from the decoder to WMPI:
1. as soon as the video decoder is stopped we reset the average - meaning we
will stop considering optimizing playback for the video
2. we have to wait for more than 4 keyframes to start optimizing video
playback, this can take seconds
3. audio decoder resets the average to 0 in pipeline's shared state

BUG=688618
TEST=Manual

Review-Url: https://codereview.chromium.org/2643103002
Cr-Commit-Position: refs/heads/master@{#444955}
(cherry picked from commit 8127a839bf18e618aaf7a8cad95440c6cd3f9f76)
==========

to

==========
[Video] Fix keyframe distance average calculations.

Found a few problems with the way the keyframe distance is calculated and
passed from the decoder to WMPI:
1. as soon as the video decoder is stopped we reset the average - meaning we
will stop considering optimizing playback for the video
2. we have to wait for more than 4 keyframes to start optimizing video
playback, this can take seconds
3. audio decoder resets the average to 0 in pipeline's shared state

BUG=688618
TEST=Manual

Review-Url: https://codereview.chromium.org/2643103002
Cr-Commit-Position: refs/heads/master@{#444955}
(cherry picked from commit 8127a839bf18e618aaf7a8cad95440c6cd3f9f76)

Review-Url: https://codereview.chromium.org/2701513002 .
Cr-Commit-Position: refs/branch-heads/2987@{#526}
Cr-Branched-From:
ad51088c0e8776e8dcd963dbe752c4035ba6dab6-refs/heads/master@{#444943}
Committed:
https://chromium.googlesource.com/chromium/src/+/7837cbd3bb82ec56e15ca5f041ad...
==========

[whywhat, 2017-02-15 19:32:27]

Committed patchset #1 (id:1) manually as
7837cbd3bb82ec56e15ca5f041ad59bf7d88a85d.