Add element_type typedef to rtc::scoped_refptr

This allows rtc::scoped_refptr to be used with templates
that use element_type as the mechanism to interface with
smart pointers.

Bug: None
Change-Id: Ie742f416a78efce0b07cfa3009d939e51506ccf9
Reviewed-on: https://webrtc-review.googlesource.com/c/115100
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26078}

api/scoped_refptr.h

@@ -71,6 +71,8 @@ namespace rtc {
 template <class T>
 class scoped_refptr {
  public:
+  typedef T element_type;
+
   scoped_refptr() : ptr_(nullptr) {}
 
   scoped_refptr(T* p) : ptr_(p) {  // NOLINT(runtime/explicit)