Initial rate allocation should not use fps = 0

A recent cl (https://codereview.webrtc.org/2510583002) introduced an
issue where the initial rate allocation (call to VideoBitrateAllocator
and any associated temporal layers) uses framerate = 0 fps. This may
cause issues, including having the rate control in ScreenshareLayers
ramp up too slowly.

This CL make the initial call use VideoCodec.maxFramerate as framerate.
Also expanded unit tests.

BUG=webrtc:6301

Review-Url: https://codereview.webrtc.org/2513383002
Cr-Commit-Position: refs/heads/master@{#15166}

webrtc/modules/video_coding/video_coding_impl.h

@@ -40,6 +40,8 @@ namespace vcm {
 
 class VCMProcessTimer {
  public:
+  static const int64_t kDefaultProcessIntervalMs = 1000;
+
   VCMProcessTimer(int64_t periodMs, Clock* clock)
       : _clock(clock),
         _periodMs(periodMs),