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/trackmediainfomap_unittest.cc b/pc/trackmediainfomap_unittest.cc
index 9f25646..65e75c4 100644
--- a/pc/trackmediainfomap_unittest.cc
+++ b/pc/trackmediainfomap_unittest.cc
@@ -35,7 +35,7 @@
   RtpParameters params;
   for (uint32_t ssrc : ssrcs) {
     RtpEncodingParameters encoding_params;
-    encoding_params.ssrc = rtc::Optional<uint32_t>(ssrc);
+    encoding_params.ssrc = ssrc;
     params.encodings.push_back(encoding_params);
   }
   return params;