Reland of "Create rtc::AtomicInt POD struct."

Relands https://codereview.webrtc.org/1420043008/ with brace initializers
instead of constructors hoping that they won't introduce static
initializers.

BUG=
R=tommi@webrtc.org

Review URL: https://codereview.webrtc.org/1498953002 .

Cr-Commit-Position: refs/heads/master@{#10920}
diff --git a/webrtc/base/criticalsection.h b/webrtc/base/criticalsection.h
index ddbf857..ed3998b 100644
--- a/webrtc/base/criticalsection.h
+++ b/webrtc/base/criticalsection.h
@@ -106,7 +106,7 @@
 
   void Unlock() UNLOCK_FUNCTION();
 
-  volatile int lock_acquired;
+  AtomicInt lock_acquired;
 };
 
 class GlobalLock : public GlobalLockPod {