Make it possible for tests to set up trace event handlers.

webrtc/rtc_base/trace_event.h

Index: webrtc/rtc_base/trace_event.h
diff --git a/webrtc/rtc_base/trace_event.h b/webrtc/rtc_base/trace_event.h
index 98f6afae943f3a2095e2bf0d6fcaeea3ba6f9a8e..6a1476d9170a493cee273063268468121af2958c 100644
--- a/webrtc/rtc_base/trace_event.h
+++ b/webrtc/rtc_base/trace_event.h
@@ -575,9 +575,15 @@
 #define INTERNAL_TRACE_EVENT_UID(name_prefix) \
     INTERNAL_TRACE_EVENT_UID2(name_prefix, __LINE__)
 
+#if WEBRTC_NON_STATIC_TRACE_EVENT_HANDLERS
+#define INTERNAL_TRACE_EVENT_INFO_TYPE const unsigned char*
+#else
+#define INTERNAL_TRACE_EVENT_INFO_TYPE static const unsigned char*
+#endif  // WEBRTC_NON_STATIC_TRACE_EVENT_HANDLERS
+
 // Implementation detail: internal macro to create static category.
 #define INTERNAL_TRACE_EVENT_GET_CATEGORY_INFO(category) \
-    static const unsigned char* INTERNAL_TRACE_EVENT_UID(catstatic) = \
+    INTERNAL_TRACE_EVENT_INFO_TYPE INTERNAL_TRACE_EVENT_UID(catstatic) = \
         TRACE_EVENT_API_GET_CATEGORY_ENABLED(category);
 
 // Implementation detail: internal macro to create static category and add