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}

webrtc/api/datachannelinterface.h

@@ -75,7 +75,7 @@ class DataChannelObserver {
   //  A data buffer was successfully received.
   virtual void OnMessage(const DataBuffer& buffer) = 0;
   // The data channel's buffered_amount has changed.
-  virtual void OnBufferedAmountChange(uint64_t previous_amount){};
+  virtual void OnBufferedAmountChange(uint64_t previous_amount) {}
 
  protected:
   virtual ~DataChannelObserver() {}