Optional: Use nullopt and implicit construction in /pc

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.

TBR=pthatcher@webrtc.org

Bug: None
Change-Id: If41c462dc3ddff664d0b70d249d760e2ca4c8ab3
Reviewed-on: https://webrtc-review.googlesource.com/23576
Commit-Queue: Oskar Sundbom <ossu@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20820}
diff --git a/pc/channel.cc b/pc/channel.cc
index 544ef4f..8bb37d1 100644
--- a/pc/channel.cc
+++ b/pc/channel.cc
@@ -1667,7 +1667,7 @@
   RtpSendParametersFromMediaDescription(audio, rtp_header_extensions,
       &send_params);
   if (audio->agc_minus_10db()) {
-    send_params.options.adjust_agc_delta = rtc::Optional<int>(kAgcMinus10db);
+    send_params.options.adjust_agc_delta = kAgcMinus10db;
   }
 
   bool parameters_applied = media_channel()->SetSendParameters(send_params);