Optional: Use nullopt and implicit construction in /modules/video_coding

Changes places where we explicitly construct an Optional to instead use
nullopt or the requisite value type only.

This CL was uploaded by git cl split.

Bug: None
Change-Id: Iedebf4dc56a973306e7d7e7649525879808dc72b
Reviewed-on: https://webrtc-review.googlesource.com/23578
Commit-Queue: Oskar Sundbom <ossu@webrtc.org>
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20878}
diff --git a/modules/video_coding/generic_decoder.cc b/modules/video_coding/generic_decoder.cc
index f3af20c..a7d6a85 100644
--- a/modules/video_coding/generic_decoder.cc
+++ b/modules/video_coding/generic_decoder.cc
@@ -58,8 +58,8 @@
                                          int64_t decode_time_ms) {
   Decoded(decodedImage,
           decode_time_ms >= 0 ? rtc::Optional<int32_t>(decode_time_ms)
-                              : rtc::Optional<int32_t>(),
-          rtc::Optional<uint8_t>());
+                              : rtc::nullopt,
+          rtc::nullopt);
   return WEBRTC_VIDEO_CODEC_OK;
 }
 
@@ -85,8 +85,7 @@
 
   const int64_t now_ms = _clock->TimeInMilliseconds();
   if (!decode_time_ms) {
-    decode_time_ms =
-        rtc::Optional<int32_t>(now_ms - frameInfo->decodeStartTimeMs);
+    decode_time_ms = now_ms - frameInfo->decodeStartTimeMs;
   }
   _timing->StopDecodeTimer(decodedImage.timestamp(), *decode_time_ms, now_ms,
                            frameInfo->renderTimeMs);