Change return type of of ReceiveStatistics::Create to unique_ptr.

There are currently three overloads with different number of arguments,
and one of those return a raw pointer. This cl changes that to unique_ptr.
The transition plan is to update those downstream call sites that
currently require a raw pointer to use one of the other overloads.

Bug: webrtc:10679
Change-Id: I234605e99c04a59fbe6f478581ed8edd96a9b05a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/148447
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28804}

modules/rtp_rtcp/include/receive_statistics.h

@@ -60,8 +60,8 @@ class ReceiveStatistics : public ReceiveStatisticsProvider,
  public:
   ~ReceiveStatistics() override = default;
 
-  static ReceiveStatistics* Create(Clock* clock) {
-    return Create(clock, nullptr).release();
+  static std::unique_ptr<ReceiveStatistics> Create(Clock* clock) {
+    return Create(clock, nullptr);
   }
 
   static std::unique_ptr<ReceiveStatistics> Create(