Revert "Cleanup of RTP references in GoogCC implementation."

This reverts commit fa79081dca9faa8322943641352d9d2fd1b1b445.

Reason for revert: Breaks downstream project.

Original change's description:
> Cleanup of RTP references in GoogCC implementation.
> 
> As the send time congestion controller now has been removed,
> we don't need the RTP related constructs anymore.
> 
> Bug: webrtc:9510
> Change-Id: I02c059ed8ae907ab4672d183c5639ad459b581aa
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/142221
> Commit-Queue: Sebastian Jansson <srte@webrtc.org>
> Reviewed-by: Björn Terelius <terelius@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#28330}

TBR=terelius@webrtc.org,srte@webrtc.org

Change-Id: I562365fc5d1da68326d603338ccc6371114d7e12
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:9510
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/143164
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28331}

modules/congestion_controller/goog_cc/probe_controller.h

@@ -26,6 +26,8 @@
 
 namespace webrtc {
 
+class Clock;
+
 struct ProbeControllerConfig {
   explicit ProbeControllerConfig(const WebRtcKeyValueConfig* key_value_config);
   ProbeControllerConfig(const ProbeControllerConfig&);