measure decode time in TimeDelta instead of ms

increasing precision since summing up rounded values leads to
a rounding error, in particular for small frames which take very
little time to decode.

BUG=webrtc:12526,webrtc:13756

Change-Id: I647c702808856a002c746ed9f115aa9bcaddc1f3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/262810
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
Cr-Commit-Position: refs/heads/main@{#37249}

modules/video_coding/generic_decoder_unittest.cc

@@ -33,7 +33,7 @@ class ReceiveCallback : public VCMReceiveCallback {
  public:
   int32_t FrameToRender(VideoFrame& videoFrame,  // NOLINT
                         absl::optional<uint8_t> qp,
-                        int32_t decode_time_ms,
+                        TimeDelta decode_time,
                         VideoContentType content_type) override {
     {
       MutexLock lock(&lock_);