Reject the descriptions that attempt to change the order of m= sections
in current local description.

When setting the descriptions, the order of m= sections would be compared
against existing m= sections and an error would be returned if the order
doesn't match.

Previously reviewed on: https://codereview.webrtc.org/3012313002/

BUG=chromium:763842
TBR=deadbeef@webrtc.org

Change-Id: I577e3424830b0a4c5ecd5524923873e30ad23d43
Reviewed-on: https://webrtc-review.googlesource.com/1200
Commit-Queue: Zhi Huang <zhihuang@webrtc.org>
Reviewed-by: Zhi Huang <zhihuang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#19842}