Add ability to disable decode in VideoProcessor.

Bug: webrtc:8448
Change-Id: Iabbf2fa0238b868c5f3869eb0ca542ffa9df7386
Reviewed-on: https://webrtc-review.googlesource.com/61660
Commit-Queue: Rasmus Brandt <brandtr@webrtc.org>
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22429}

modules/video_coding/codecs/test/test_config.h

@@ -66,6 +66,9 @@ struct TestConfig {
   // Bitstream constraints.
   size_t max_payload_size_bytes = 1440;
 
+  // Should we decode the encoded frames?
+  bool decode = true;
+
   // Force the encoder and decoder to use a single core for processing.
   bool use_single_core = false;