Fix a few warnings that -Wnull-conversion of a future clang will complain about.

base/mac/scoped_ioobject.h

Index: base/mac/scoped_ioobject.h
diff --git a/base/mac/scoped_ioobject.h b/base/mac/scoped_ioobject.h
index d5cb90cf6e1641e9547550c350b982d1f79e1503..f389cc040324c2f5c8f70df4aff57bb8315fc493 100644
--- a/base/mac/scoped_ioobject.h
+++ b/base/mac/scoped_ioobject.h
@@ -20,7 +20,7 @@ class ScopedIOObject {
  public:
   typedef IOT element_type;
 
-  explicit ScopedIOObject(IOT object = NULL)
+  explicit ScopedIOObject(IOT object = IO_OBJECT_NULL)
       : object_(object) {
   }
 
@@ -29,7 +29,7 @@ class ScopedIOObject {
       IOObjectRelease(object_);
   }
 
-  void reset(IOT object = NULL) {
+  void reset(IOT object = IO_OBJECT_NULL) {
     if (object_)
       IOObjectRelease(object_);
     object_ = object;
@@ -59,7 +59,7 @@ class ScopedIOObject {
 
   IOT release() WARN_UNUSED_RESULT {
     IOT temp = object_;
-    object_ = NULL;
+    object_ = IO_OBJECT_NULL;
     return temp;
   }