Turn MoveLeave event sending into a feature for testing enabling it.

ui/events/blink/blink_features.h

Index: ui/events/blink/blink_features.h
diff --git a/ui/events/blink/blink_features.h b/ui/events/blink/blink_features.h
index 5f1d848bc76851419a03cb37377624dbeb1e928a..3d1473d0780d8f7576b8bcc655b0a9ca3ad8ab50 100644
--- a/ui/events/blink/blink_features.h
+++ b/ui/events/blink/blink_features.h
@@ -11,6 +11,11 @@ namespace features {
 
 extern const base::Feature kVsyncAlignedInputEvents;
 
+// This feature allows native ET_MOUSE_EXIT events to be passed
+// through to blink as mouse leave events. Traditionally these events were
+// converted to mouse move events due to a number of incosistencies on
+// the native platforms. Enabled by default on ChromeOS. crbug.com/450631
+extern const base::Feature kSendMouseLeaveEvents;
 }
 
 #endif  // UI_EVENTS_BLINK_BLINK_FEATURES_H_