Reland of Periodically update codec bit/frame rate settings.

Patch set 1 is a reland + trivial rebase.
Patch set >= 2 contains bug fixes.

> Original issue's description:
> > Fix bug in vie_encoder.cc which caused channel parameters not to be updated at regular intervals, as it was intended.
> >
> > That however exposes a bunch of failed test, so this CL also fixed a few other things:
> > * FakeEncoder should trust the configured FPS value rather than guesstimating itself based on the realtime clock, so as not to completely undershoot targets in offline mode. Also, compensate for key-frame overshoots when outputting delta frames.
> > * FrameDropper should not assuming incoming frame rate is 0 if no frames have been seen.
> > * Fix a bunch of test cases that started failing because they were relying on the fake encoder undershooting.
> > * Fix test
> >
> > BUG=7664
> >
> > Review-Url: https://codereview.webrtc.org/2883963002
> > Cr-Commit-Position: refs/heads/master@{#18473}
> > Committed: https://chromium.googlesource.com/external/webrtc/+/6431e21da672a5f3bbf166d3d4d98b171d015706

BUG=webrtc:7664

Review-Url: https://codereview.webrtc.org/2953053002
Cr-Commit-Position: refs/heads/master@{#18782}
diff --git a/webrtc/modules/video_coding/media_optimization.cc b/webrtc/modules/video_coding/media_optimization.cc
index ccd23f6..f0cbd62 100644
--- a/webrtc/modules/video_coding/media_optimization.cc
+++ b/webrtc/modules/video_coding/media_optimization.cc
@@ -118,7 +118,13 @@
   // Update encoding rates following protection settings.
   float target_video_bitrate_kbps =
       static_cast<float>(video_target_bitrate_) / 1000.0f;
-  frame_dropper_->SetRates(target_video_bitrate_kbps, incoming_frame_rate_);
+  float framerate = incoming_frame_rate_;
+  if (framerate == 0.0) {
+    // No framerate estimate available, use configured max framerate instead.
+    framerate = user_frame_rate_;
+  }
+
+  frame_dropper_->SetRates(target_video_bitrate_kbps, framerate);
 
   return video_target_bitrate_;
 }