Add default constructor for rtc::Event

Bug: webrtc:9962
Change-Id: Icaa91e657e6881fcb1553f354c07866109a0ea68
Reviewed-on: https://webrtc-review.googlesource.com/c/109500
Commit-Queue: Niels Moller <nisse@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25535}

modules/video_coding/frame_buffer2_unittest.cc

@@ -137,9 +137,7 @@ class TestFrameBuffer2 : public ::testing::Test {
                                 &stats_callback_)),
         rand_(0x34678213),
         tear_down_(false),
-        extract_thread_(&ExtractLoop, this, "Extract Thread"),
-        trigger_extract_event_(false, false),
-        crit_acquired_event_(false, false) {}
+        extract_thread_(&ExtractLoop, this, "Extract Thread") {}
 
   void SetUp() override { extract_thread_.Start(); }