Aura: Allow browser UI thread waiting for compositor commits.

cc/completion_event.h

Index: cc/completion_event.h
diff --git a/cc/completion_event.h b/cc/completion_event.h
index 462e4ba12f6885a40c01711e66187a2e795c4d52..795b0303323fb9db75f2444346e860107ecf2944 100644
--- a/cc/completion_event.h
+++ b/cc/completion_event.h
@@ -6,6 +6,7 @@
 #define CCCompletionEvent_h
 
 #include "base/synchronization/waitable_event.h"
+#include "base/threading/thread_restrictions.h"
 
 namespace cc {
 
@@ -36,6 +37,7 @@ public:
 #ifndef NDEBUG
         m_waited = true;
 #endif
+        base::ThreadRestrictions::ScopedAllowWait allow_wait;
         m_event.Wait();
     }