Add ScopedAllowBaseSyncPrimitives to WebRTC for compatibility with Chromium.

This will be overridden in Chromium in a future CL and following that,
used inside of WebRTC as needed while issues with blocking events are
being fixed.

Bug: chromium:796889, chromium:795340
Change-Id: I098a03d9bad621f1349ef483b82e4d8e786a8a75
Reviewed-on: https://webrtc-review.googlesource.com/40140
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Commit-Queue: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21659}

rtc_base/event.h

@@ -49,6 +49,20 @@ class Event {
   RTC_DISALLOW_IMPLICIT_CONSTRUCTORS(Event);
 };
 
+// This class is provided for compatibility with Chromium.
+// The rtc::Event implementation is overriden inside of Chromium for the
+// purposes of detecting when threads are blocked that shouldn't be as well as
+// to use the more accurate event implementation that's there than is provided
+// by default on some platforms (e.g. Windows).
+// When building with standalone WebRTC, this class is a noop.
+// For further information, please see the ScopedAllowBaseSyncPrimitives class
+// in Chromium.
+class ScopedAllowBaseSyncPrimitives {
+ public:
+  ScopedAllowBaseSyncPrimitives() {}
+  ~ScopedAllowBaseSyncPrimitives() {}
+};
+
 }  // namespace rtc
 
 #endif  // RTC_BASE_EVENT_H_