Add a deprecation message to rtcpMuxPolicy of negotiate.

Add a deprecation message to rtcpMuxPolicy of negotiate.

When parsing the RTCConfiguration, if the RtcpMuxPolicy is explicitly set
to be "negotiate", the deprecation message will be return to the console.

If this is approved, the change will be merged to M57.

BUG=chromium:685727
Review-Url: https://codereview.chromium.org/2720763004
Cr-Commit-Position: refs/heads/master@{#453771}
(cherry picked from commit 918bb2c91f07a2cce51cea68554bd56566dec0dc)

Review-Url: https://codereview.chromium.org/2725293004 .
Cr-Commit-Position: refs/branch-heads/2987@{#750}
Cr-Branched-From: ad51088c0e8776e8dcd963dbe752c4035ba6dab6-refs/heads/master@{#444943}
Committed: https://chromium.googlesource.com/chromium/src/+/f820422f2b7196ef6c410558b4fdc50ad7f613ad

[foolip, 2017-03-03 15:28:15]

Description was changed from

==========
Add a deprecation message to rtcpMuxPolicy of negotiate.

When parsing the RTCConfiguration, if the RtcpMuxPolicy is explicitly set
to be "negotiate", the deprecation message will be return to the console.

If this is approved, the change will be merged to M57.

BUG=chromium:685727

Review-Url: https://codereview.chromium.org/2720763004
Cr-Commit-Position: refs/heads/master@{#453771}
(cherry picked from commit 918bb2c91f07a2cce51cea68554bd56566dec0dc)
==========

to

==========
Add a deprecation message to rtcpMuxPolicy of negotiate.

When parsing the RTCConfiguration, if the RtcpMuxPolicy is explicitly set
to be "negotiate", the deprecation message will be return to the console.

If this is approved, the change will be merged to M57.

BUG=chromium:685727

Review-Url: https://codereview.chromium.org/2720763004
Cr-Commit-Position: refs/heads/master@{#453771}
(cherry picked from commit 918bb2c91f07a2cce51cea68554bd56566dec0dc)

Review-Url: https://codereview.chromium.org/2725293004 .
Cr-Commit-Position: refs/branch-heads/2987@{#750}
Cr-Branched-From:
ad51088c0e8776e8dcd963dbe752c4035ba6dab6-refs/heads/master@{#444943}
Committed:
https://chromium.googlesource.com/chromium/src/+/f820422f2b7196ef6c410558b4fd...
==========

[foolip, 2017-03-03 15:28:16]

Committed patchset #1 (id:1) manually as
f820422f2b7196ef6c410558b4fdc50ad7f613ad.