Optional: Use nullopt and implicit construction in /api

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.

R=pthatcher@webrtc.org

Bug: None
Change-Id: Icdc7e9e4395eeac053483c69e53501e5aa222107
Reviewed-on: https://webrtc-review.googlesource.com/23567
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Peter Thatcher <pthatcher@webrtc.org>
Commit-Queue: Oskar Sundbom <ossu@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20867}
diff --git a/api/mediaconstraintsinterface.cc b/api/mediaconstraintsinterface.cc
index 2894f5b..90a957c 100644
--- a/api/mediaconstraintsinterface.cc
+++ b/api/mediaconstraintsinterface.cc
@@ -67,7 +67,7 @@
   T value;
   bool present = FindConstraint<T>(constraints, key, &value, nullptr);
   if (present) {
-    *value_out = rtc::Optional<T>(value);
+    *value_out = value;
   }
 }
 }  // namespace
@@ -268,7 +268,7 @@
   // When |kAudioNetworkAdaptorConfig| is defined, it both means that audio
   // network adaptor is desired, and provides the config string.
   if (options->audio_network_adaptor_config) {
-    options->audio_network_adaptor = rtc::Optional<bool>(true);
+    options->audio_network_adaptor = true;
   }
 }
 
diff --git a/api/video_codecs/video_encoder.cc b/api/video_codecs/video_encoder.cc
index e3cc516..e1cc0cf 100644
--- a/api/video_codecs/video_encoder.cc
+++ b/api/video_codecs/video_encoder.cc
@@ -61,15 +61,14 @@
 }
 
 VideoEncoder::ScalingSettings::ScalingSettings(bool on, int low, int high)
-    : enabled(on),
-      thresholds(rtc::Optional<QpThresholds>(QpThresholds(low, high))) {}
+    : enabled(on), thresholds(QpThresholds(low, high)) {}
 
 VideoEncoder::ScalingSettings::ScalingSettings(bool on,
                                                int low,
                                                int high,
                                                int min_pixels)
     : enabled(on),
-      thresholds(rtc::Optional<QpThresholds>(QpThresholds(low, high))),
+      thresholds(QpThresholds(low, high)),
       min_pixels_per_frame(min_pixels) {}
 
 VideoEncoder::ScalingSettings::ScalingSettings(bool on, int min_pixels)