Enable cpplint and fix cpplint errors in webrtc/api

Adding 'explicit' to these constructors has a low risk of causing
compatibility problems:
explicit RTCConfiguration(RTCConfigurationType type)
explicit IdBase(StatsType type)

BUG=webrtc:5267
TESTED=Fixed issues reported by:
find webrtc/api -type f -name *.cc -o -name *.h | xargs cpplint.py
followed by 'git cl presubmit'.

Review-Url: https://codereview.webrtc.org/2663063003
Cr-Commit-Position: refs/heads/master@{#16392}
diff --git a/webrtc/api/mediaconstraintsinterface.cc b/webrtc/api/mediaconstraintsinterface.cc
index 5361a28..f393a2e 100644
--- a/webrtc/api/mediaconstraintsinterface.cc
+++ b/webrtc/api/mediaconstraintsinterface.cc
@@ -70,7 +70,7 @@
     *value_out = rtc::Optional<T>(value);
   }
 }
-}
+}  // namespace
 
 namespace webrtc {