Add missing #include + dependency to weak_ptr_unittest.
I ran into this while looking at the rtc::Event class.

TBR=mbonadei@webrtc.org

Bug: webrtc:8596
Change-Id: Iee00b15381548b69f6cb9485788c787bde23494a
Reviewed-on: https://webrtc-review.googlesource.com/28280
Reviewed-by: Tommi <tommi@webrtc.org>
Commit-Queue: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20981}
diff --git a/rtc_base/weak_ptr_unittest.cc b/rtc_base/weak_ptr_unittest.cc
index c22032b..6b0b452 100644
--- a/rtc_base/weak_ptr_unittest.cc
+++ b/rtc_base/weak_ptr_unittest.cc
@@ -10,6 +10,7 @@
 
 #include <string>
 
+#include "rtc_base/event.h"
 #include "rtc_base/gunit.h"
 #include "rtc_base/task_queue.h"
 #include "rtc_base/weak_ptr.h"