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/mediaconstraintsinterface_unittest.cc b/pc/mediaconstraintsinterface_unittest.cc
index 21c547a..882d097 100644
--- a/pc/mediaconstraintsinterface_unittest.cc
+++ b/pc/mediaconstraintsinterface_unittest.cc
@@ -62,7 +62,7 @@
   // values that are already present.
   constraints = FakeConstraints();
   configuration = old_configuration;
-  configuration.enable_dtls_srtp = rtc::Optional<bool>(true);
+  configuration.enable_dtls_srtp = true;
   configuration.audio_jitter_buffer_max_packets = 34;
   CopyConstraintsIntoRtcConfiguration(&constraints, &configuration);
   EXPECT_EQ(34, configuration.audio_jitter_buffer_max_packets);