RTC_EXPORT webrtc::RTCNonStandardStatsMember<T>.

Since webrtc::RTCNonStandardStatsMember<T> extends some explicit
template initializations, its symbols need to be exported as well.

Example of an lld-link error:
https://ci.chromium.org/p/chromium/builders/try/win_chromium_compile_dbg_ng/435974

Bug: webrtc:9419
Change-Id: I48bd484b821b17a214e0180e78ba34c38e247947
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/159693
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29780}

api/stats/rtc_stats.h

@@ -387,7 +387,7 @@ WEBRTC_DECLARE_RTCSTATSMEMBER(std::vector<std::string>);
 // Using inheritance just so that it's obvious from the member's declaration
 // whether it's standardized or not.
 template <typename T>
-class RTCNonStandardStatsMember : public RTCStatsMember<T> {
+class RTC_EXPORT RTCNonStandardStatsMember : public RTCStatsMember<T> {
  public:
   explicit RTCNonStandardStatsMember(const char* name)
       : RTCStatsMember<T>(name) {}