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/stats/rtcstats_objects.h b/webrtc/api/stats/rtcstats_objects.h
index c5277e7..ac00386 100644
--- a/webrtc/api/stats/rtcstats_objects.h
+++ b/webrtc/api/stats/rtcstats_objects.h
@@ -12,6 +12,7 @@
 #define WEBRTC_API_STATS_RTCSTATS_OBJECTS_H_
 
 #include <string>
+#include <vector>
 
 #include "webrtc/api/stats/rtcstats.h"