Prepare VideoProcessor for async simulcast support.

* Add support for SimulcastEncoderAdapter wrapping of encoder.
* Store input frame timestamps out-of-band, so we don't need to keep
  a raw VideoFrame around just for it's timestamp.
* Store current frame rate in |framerate_fps_|, instead of in
  codec settings struct.
* Add some comments and reorder some data members.
* Explicitly include VideoBitrateAllocator.
* Change type of |input_frames_|, to avoid one layer of indirection.
* Move VideoProcessor::CalculateFrameQuality to anonymous namespace.

This change should have no functional implications.

Bug: webrtc:8448
Change-Id: I10c140eeda750d9bd37bfb6cb1e8acb401fb91d3
Reviewed-on: https://webrtc-review.googlesource.com/60520
Commit-Queue: Rasmus Brandt <brandtr@webrtc.org>
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22346}

modules/video_coding/codecs/test/test_config.h

@@ -90,6 +90,9 @@ struct TestConfig {
   bool hw_encoder = false;
   bool hw_decoder = false;
 
+  // Should the encoder be wrapped in a SimulcastEncoderAdapter?
+  bool simulcast_adapted_encoder = false;
+
   // Should the hardware codecs be wrapped in software fallbacks?
   bool sw_fallback_encoder = false;
   bool sw_fallback_decoder = false;