Fix jitter buffer bug around out-of-order packets and non-RTX padding.

tl;dr - non-continuous frames (due to padding) would get stuck as incomplete if the previous complete frame arrived and was decoded before the padding arrived.This fix re-checks the incomplete frame list for continuous frames after old packets arrive.

When padding is enabled and RTX is not, padding is sent as empty RTP packets tacked onto the end of completed frames (meaning: same timestamp, but after a packet with the marker bit set). Given the following set of circumstances, codified in the new unit test method, a frame can get permanently stuck in the incomplete frames list:

- Frame A decoded (packets 94-95). Next expected sequence number is 96.
- Frame C arrives (packets 100-101) and is marked complete. It isn't continuous, since it starts at 100, so it's placed in the incomplete frame list.
- Frame B arrives (packets 96-97) and is complete, since 97 has a marker bit.  Turns out that packets 98-99 are padding, but the receiver doesn't know that.
- Frame B is decoded, removed from the decodable frames list, and last decoded state is updated.
- Packets 98-99 arrive. They hit the IsOldPacket check and update the last decoded state, but they don't trigger FindAndInsertContinuousFrames.
- Further packets/frames arrive and complete, but FindAndInsertContinuousFrames only runs on frames that are newer than the newly completed frame.

In this state, Frame C is permanently stuck as incomplete, so the jitter buffer overall is stuck until max NACK age (default: 450 packets), the max NACK list size (default: 200 packets), or a keyframe arrives and IsContinuous returns true for the keyframe.

(Before the November refactoring, Frame B wouldn't have to have been decoded for the bug to trigger; just having a complete continuous frame at any time before the padding arrived would cause this state, as FindAndInsertContinuousFrames was only called when the frame originally became continuous and was inserted into the decodable frames list. Post refactoring, the frame is removed/re-added to the decodable list on every padding packet that arrives)

BUG=
R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/50959004

Cr-Commit-Position: refs/heads/master@{#9264}

webrtc/modules/video_coding/main/source/jitter_buffer.h

@@ -214,6 +214,12 @@ class VCMJitterBuffer {
   // all decodable frames into account.
   bool IsContinuous(const VCMFrameBuffer& frame) const
       EXCLUSIVE_LOCKS_REQUIRED(crit_sect_);
+  // Looks for frames in |incomplete_frames_| which are continuous in the
+  // provided |decoded_state|. Starts the search from the timestamp of
+  // |decoded_state|.
+  void FindAndInsertContinuousFramesWithState(
+      const VCMDecodingState& decoded_state)
+      EXCLUSIVE_LOCKS_REQUIRED(crit_sect_);
   // Looks for frames in |incomplete_frames_| which are continuous in
   // |last_decoded_state_| taking all decodable frames into account. Starts
   // the search from |new_frame|.