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}

PRESUBMIT.py

@@ -15,6 +15,7 @@ import sys
 
 # Directories that will be scanned by cpplint by the presubmit script.
 CPPLINT_DIRS = [
+  'webrtc/api',
   'webrtc/audio',
   'webrtc/call',
   'webrtc/common_video',