Add config option to run VideoCodecTest in real time.

It's reasonable to allow clients implementing their own VideoCodecTests
to decide wether they should run in real-time.

Removes the IsAsyncCodec helper, as the assumptions it made are outdated,
and it is no longer useful.

Bug: None
Change-Id: If766935d4947555af54f499a30cfe553bde4c1ab
Reviewed-on: https://webrtc-review.googlesource.com/95722
Commit-Queue: Kári Helgason <kthelgason@webrtc.org>
Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24478}

modules/video_coding/codecs/test/videoprocessor.cc

@@ -444,7 +444,7 @@ void VideoProcessor::FrameEncoded(
                                config_.codec_settings.codecType));
   }
 
-  if (!config_.IsAsyncCodec()) {
+  if (!config_.encode_in_real_time) {
     // To get pure encode time for next layers, measure time spent in encode
     // callback and subtract it from encode time of next layers.
     post_encode_time_ns_ += rtc::TimeNanos() - encode_stop_ns;