Set processing time according to the decode time if provided by the decoder.

Followup to https://webrtc-review.googlesource.com/c/src/+/183880

Bug: webrtc:9106
Change-Id: Ic79e0be359ced00ffe0c52acc3c5aaceafc793de
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/186300
Commit-Queue: Philip Eliasson <philipel@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Reviewed-by: Johannes Kron <kron@webrtc.org>
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32293}

api/video/video_stream_decoder.h

@@ -31,7 +31,6 @@ class VideoStreamDecoderInterface {
     virtual ~Callbacks() = default;
 
     struct FrameInfo {
-      absl::optional<int> decode_time_ms;
       absl::optional<int> qp;
       VideoContentType content_type;
     };