Replace some usage of EventWrapper with rtc::Event.

Bug: webrtc:3380
Change-Id: Id33b19bf107273e6f838aa633784db73d02ae2c2
Reviewed-on: https://webrtc-review.googlesource.com/c/107888
Reviewed-by: Henrik Grunell <henrikg@webrtc.org>
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Reviewed-by: Per Kjellander <perkj@webrtc.org>
Reviewed-by: Oskar Sundbom <ossu@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25407}

system_wrappers/include/event_wrapper.h

@@ -12,11 +12,7 @@
 #define SYSTEM_WRAPPERS_INCLUDE_EVENT_WRAPPER_H_
 
 namespace webrtc {
-enum EventTypeWrapper {
-  kEventSignaled = 1,
-  kEventError = 2,
-  kEventTimeout = 3
-};
+enum EventTypeWrapper { kEventSignaled = 1, kEventTimeout = 2 };
 
 #define WEBRTC_EVENT_INFINITE 0xffffffff