Break up rtc_event_log_api to solve circular dependencies.

The original rtc_event_log_api is refactored to a pure API target plus
multiple targets coupled with WebRTC implementations.

Bug: None
Change-Id: Iab9eee3f7bf4228c52d94a5f26fc39bb99b5033f
Reviewed-on: https://webrtc-review.googlesource.com/43247
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Commit-Queue: Qingsi Wang <qingsi@google.com>
Cr-Commit-Position: refs/heads/master@{#21811}

logging/rtc_event_log/rtc_event_log.h

@@ -57,9 +57,7 @@ class RtcEventLog {
 class RtcEventLogNullImpl : public RtcEventLog {
  public:
   bool StartLogging(std::unique_ptr<RtcEventLogOutput> output,
-                    int64_t output_period_ms) override {
-    return false;
-  }
+                    int64_t output_period_ms) override;
   void StopLogging() override {}
   void Log(std::unique_ptr<RtcEvent> event) override {}
 };

logging/rtc_event_log/rtc_event_log_impl.cc

@@ -377,4 +377,10 @@ std::unique_ptr<RtcEventLog> RtcEventLog::CreateNull() {
   return std::unique_ptr<RtcEventLog>(new RtcEventLogNullImpl());
 }
 
+bool RtcEventLogNullImpl::StartLogging(
+    std::unique_ptr<RtcEventLogOutput> output,
+    int64_t output_period_ms) {
+  return false;
+}
+
 }  // namespace webrtc