RTC_EXPORT RTCStatsMember's specialized members.

Without this, on some build configurations the symbols of the
specialized members don't get exported as explained at:
https://bugs.chromium.org/p/chromium/issues/detail?id=1026078#c10

Bug: chromium:1026078
Change-Id: I0c3058a82d60e6de5e401dbec5bb8501b7bbd8b5
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/160046
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29846}
diff --git a/api/stats/rtc_stats.h b/api/stats/rtc_stats.h
index 5f5bb41..d45902e 100644
--- a/api/stats/rtc_stats.h
+++ b/api/stats/rtc_stats.h
@@ -359,13 +359,13 @@
   template <>                                                               \
   RTC_EXPORT RTCStatsMemberInterface::Type RTCStatsMember<T>::StaticType(); \
   template <>                                                               \
-  bool RTCStatsMember<T>::is_sequence() const;                              \
+  RTC_EXPORT bool RTCStatsMember<T>::is_sequence() const;                   \
   template <>                                                               \
-  bool RTCStatsMember<T>::is_string() const;                                \
+  RTC_EXPORT bool RTCStatsMember<T>::is_string() const;                     \
   template <>                                                               \
-  std::string RTCStatsMember<T>::ValueToString() const;                     \
+  RTC_EXPORT std::string RTCStatsMember<T>::ValueToString() const;          \
   template <>                                                               \
-  std::string RTCStatsMember<T>::ValueToJson() const;                       \
+  RTC_EXPORT std::string RTCStatsMember<T>::ValueToJson() const;            \
   extern template class RTC_EXPORT_TEMPLATE_DECLARE(RTC_EXPORT)             \
       RTCStatsMember<T>