Adding a factory to remote bitrate estimator and allow it to be set via config.

Additionally:
 - clean api to set remote bitrate estimator mode.
 - clean api to set over use detector options.

R=mflodman@webrtc.org, stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/1448006

git-svn-id: http://webrtc.googlecode.com/svn/trunk@4027 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/modules/rtp_rtcp/source/rtcp_sender_unittest.cc

@@ -277,9 +277,7 @@ class RtcpSenderTest : public ::testing::Test {
         system_clock_(Clock::GetRealTimeClock()),
         remote_bitrate_observer_(),
         remote_bitrate_estimator_(
-            RemoteBitrateEstimator::Create(
-                over_use_detector_options_,
-                RemoteBitrateEstimator::kSingleStreamEstimation,
+            RemoteBitrateEstimatorFactory().Create(
                 &remote_bitrate_observer_,
                 system_clock_)) {
     test_transport_ = new TestTransport();