Add support for padding in pacer.

This improves pacer-based padding by making sure it limits padding according to:
- Never pad more than 800 kbps.
- Padding + media should not go above a given target bitrate.

Also adds appropriate unittests to make sure we reach the given targets.

BUG=1837
R=mflodman@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/1582005

git-svn-id: http://webrtc.googlecode.com/svn/trunk@4168 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/modules/video_coding/main/interface/video_coding.h

@@ -165,7 +165,7 @@ public:
     //                     < 0,                     on error.
     virtual int32_t CodecConfigParameters(uint8_t* buffer, int32_t size) = 0;
 
-    // API to get currently configured encoder target bitrate in kbit/s.
+    // API to get currently configured encoder target bitrate in bits/s.
     //
     // Return value      : 0,   on success.
     //                     < 0, on error.