Revert of Change type of pid_diff (int16_t -> uint8_t) according to updates in RTP payload profile.

Revert of Change type of pid_diff (int16_t -> uint8_t) according to updates in RTP payload profile. (patchset #3 id:40001 of https://codereview.webrtc.org/1427253002/ )

Reason for revert:
Breaks bot.

Original issue's description:
> Change type of pid_diff (int16_t -> uint8_t) according to updates in RTP payload profile. Max p_diff is 8 bits.
>
> Change type of number of reference pictures (size_t -> uint8_t). Max is 2 bits.
>
> Size of WebRtcRTPHeader: 4352 -> 1784 bytes.
>
> BUG=webrtc:5144, chromium:500602
>
> Committed: https://crrev.com/81c5c7f8157f767747bd97419eb0a589207354cf
> Cr-Commit-Position: refs/heads/master@{#10504}

TBR=stefan@webrtc.org,mflodman@webrtc.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:5144, chromium:500602
Committed: https://crrev.com/5d4e944391043dddc36fc3d5570a34be1b286a5a
Cr-Commit-Position: refs/heads/master@{#10508}

[åsapersson, 2015-11-04 15:40:20]

Created Revert of Change type of pid_diff (int16_t -> uint8_t) according to
updates in RTP payload profile.

[commit-bot: I haz the power, 2015-11-04 15:40:26]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1423493005/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1423493005/1

[commit-bot: I haz the power, 2015-11-04 15:40:39]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-11-04 15:40:53]

Patchset 1 (id:??) landed as
https://crrev.com/5d4e944391043dddc36fc3d5570a34be1b286a5a
Cr-Commit-Position: refs/heads/master@{#10508}