Update encoder settings periodically, not only on new bandwidth estimate

Also moved actual update call to encoder thread, and tweaked frame rate
estimate to be less noisy.

This is a re-upload of https://review.webrtc.org/47249004

BUG=chromium:476469
R=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1180623005.

Cr-Commit-Position: refs/heads/master@{#9417}

webrtc/modules/video_coding/main/source/media_optimization.cc

@@ -631,8 +631,9 @@ void MediaOptimization::ProcessIncomingFrameRate(int64_t now) {
     }
   }
   if (num > 1) {
-    const int64_t diff = now - incoming_frame_times_[num - 1];
-    incoming_frame_rate_ = 1.0;
+    const int64_t diff =
+        incoming_frame_times_[0] - incoming_frame_times_[num - 1];
+    incoming_frame_rate_ = 0.0;  // No frame rate estimate available.
     if (diff > 0) {
       incoming_frame_rate_ = nr_of_frames * 1000.0f / static_cast<float>(diff);
     }