Export more symbols to fix Chromecast component build

When building certain Chromecast build flavors in component build mode,
there are some link errors due to symbols not being exported. This CL
fixes those issues.

TBR: kwiberg@webrtc.org
Bug: None
Change-Id: I408f0a84b8ac4610cc6b5aa6ff58248ea82c9c66
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/161148
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29992}
diff --git a/api/peer_connection_interface.h b/api/peer_connection_interface.h
index d118e52..72e20b9 100644
--- a/api/peer_connection_interface.h
+++ b/api/peer_connection_interface.h
@@ -1339,7 +1339,8 @@
 // of networking classes, it should use the alternate
 // CreatePeerConnectionFactory method which accepts threads as input, and use
 // the CreatePeerConnection version that takes a PortAllocator as an argument.
-class PeerConnectionFactoryInterface : public rtc::RefCountInterface {
+class RTC_EXPORT PeerConnectionFactoryInterface
+    : public rtc::RefCountInterface {
  public:
   class Options {
    public: