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.h

@@ -79,6 +79,7 @@ class MediaOptimization {
   // Informs Media Optimization of encoded output.
   int32_t UpdateWithEncodedData(const EncodedImage& encoded_image);
 
+  // InputFrameRate 0 = no frame rate estimate available.
   uint32_t InputFrameRate();
   uint32_t SentFrameRate();
   uint32_t SentBitRate();