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/proxy.h b/webrtc/api/proxy.h
index 51cdd29..f834790 100644
--- a/webrtc/api/proxy.h
+++ b/webrtc/api/proxy.h
@@ -52,6 +52,7 @@
 #define WEBRTC_API_PROXY_H_
 
 #include <memory>
+#include <utility>
 
 #include "webrtc/base/event.h"
 #include "webrtc/base/thread.h"
@@ -349,6 +350,7 @@
   T5 a5_;
 };
 
+
 // Helper macros to reduce code duplication.
 #define PROXY_MAP_BOILERPLATE(c)                                \
   template <class INTERNAL_CLASS>                               \
@@ -363,6 +365,9 @@
     const INTERNAL_CLASS* internal() const { return c_.get(); } \
     INTERNAL_CLASS* internal() { return c_.get(); }
 
+#define END_PROXY_MAP() \
+  };
+
 #define SIGNALING_PROXY_MAP_BOILERPLATE(c)                               \
  protected:                                                              \
   c##ProxyWithInternal(rtc::Thread* signaling_thread, INTERNAL_CLASS* c) \
@@ -448,13 +453,13 @@
  private:                                                              \
   rtc::Thread* destructor_thread() const { return signaling_thread_; } \
                                                                        \
- public:
+ public:  // NOLINTNEXTLINE
 
 #define PROXY_WORKER_THREAD_DESTRUCTOR()                            \
  private:                                                           \
   rtc::Thread* destructor_thread() const { return worker_thread_; } \
                                                                     \
- public:
+ public:  // NOLINTNEXTLINE
 
 #define PROXY_METHOD0(r, method)                           \
   r method() override {                                    \
@@ -549,10 +554,6 @@
     return call.Marshal(RTC_FROM_HERE, worker_thread_);                      \
   }
 
-#define END_PROXY_MAP() \
-  }                     \
-  ;
-
 }  // namespace webrtc
 
 #endif  //  WEBRTC_API_PROXY_H_