Delete Start and Stop methods from TestVideoCapturer.

Preparation for replacing use of TestVideoCapturer as an interface,
instead using VideoSourceInterface.

Methods kept as non-virtual on the subclass FrameGeneratorCapturer,
but it's changed to be started on creation.

Bug: webrtc:6353
Change-Id: Iae1c9a0ee55d730d4992204f62227ef2f057d58e
Reviewed-on: https://webrtc-review.googlesource.com/c/114425
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Per Kjellander <perkj@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26037}

pc/test/framegeneratorcapturervideotracksource.h

@@ -49,12 +49,10 @@ class FrameGeneratorCapturerVideoTrackSource : public VideoTrackSource {
   ~FrameGeneratorCapturerVideoTrackSource() = default;
 
   void Start() {
-    video_capturer_->Start();
     SetState(kLive);
   }
 
   void Stop() {
-    video_capturer_->Stop();
     SetState(kMuted);
   }