Avoid to unconditionally include rtc_base/win32.h.

This CL adds #error to spot where rtc_base/win32.h is unconditionally
included and fixes all the places where it happens.

Bug: webrtc:8814
Change-Id: I3c005acf2cdb58a51f1bcaa4acaeebd272c56660
Reviewed-on: https://webrtc-review.googlesource.com/46060
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21840}
diff --git a/examples/peerconnection/client/main_wnd.h b/examples/peerconnection/client/main_wnd.h
index f3ed2f5..8edf81f 100644
--- a/examples/peerconnection/client/main_wnd.h
+++ b/examples/peerconnection/client/main_wnd.h
@@ -20,7 +20,9 @@
 #include "examples/peerconnection/client/peer_connection_client.h"
 #include "media/base/mediachannel.h"
 #include "media/base/videocommon.h"
+#if defined(WEBRTC_WIN)
 #include "rtc_base/win32.h"
+#endif  // WEBRTC_WIN
 
 class MainWndCallback {
  public: