Change the low-bitrate handling in BitrateControllerImpl

Changing to using strategy classes rather than having two different
derived classes of BitrateControllerImpl. This enables run-time switching
of the strategy, which is now possible through a new API. The reason is
that it must fit the current design of ViE.

BUG=2436
R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/2789004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@5028 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/modules/bitrate_controller/include/bitrate_controller.h

@@ -70,6 +70,9 @@ class BitrateController {
                                   const uint32_t max_bitrate) = 0;
 
   virtual void RemoveBitrateObserver(BitrateObserver* observer) = 0;
+
+  // Changes the mode that was set in the constructor.
+  virtual void EnforceMinBitrate(bool enforce_min_bitrate) = 0;
 };
 }  // namespace webrtc
 #endif  // WEBRTC_MODULES_BITRATE_CONTROLLER_INCLUDE_BITRATE_CONTROLLER_H_