Fix chrome tests by making SetBitrateAllocationStrategy not pure virtual

Bug: webrtc:8243
Change-Id: I6fedf00d6735895771d460299458c5f8b6face8d
Reviewed-on: https://webrtc-review.googlesource.com/13760
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Commit-Queue: Alex Narest <alexnarest@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20351}

api/peerconnectioninterface.h

@@ -789,9 +789,10 @@ class PeerConnectionInterface : public rtc::RefCountInterface {
   // Sets current strategy. If not set default WebRTC allocator will be used.
   // May be changed during an active session. The strategy
   // ownership is passed with std::unique_ptr
+  // TODO(alexnarest): Make this pure virtual when tests will be updated
   virtual void SetBitrateAllocationStrategy(
       std::unique_ptr<rtc::BitrateAllocationStrategy>
-          bitrate_allocation_strategy) = 0;
+          bitrate_allocation_strategy) {}
 
   // Returns the current SignalingState.
   virtual SignalingState signaling_state() = 0;