Make base::internal::CallbackBase::is_null inline.

base/callback_internal.h

Index: base/callback_internal.h
diff --git a/base/callback_internal.h b/base/callback_internal.h
index e66623cc513f9853e8d7baf3dee2936bdac22ea5..9d7761c409781ac613340b26cca474cea5c148a2 100644
--- a/base/callback_internal.h
+++ b/base/callback_internal.h
@@ -37,7 +37,7 @@ class BindStateBase : public RefCountedThreadSafe<BindStateBase> {
 class BASE_EXPORT CallbackBase {
  public:
   // Returns true if Callback is null (doesn't refer to anything).
-  bool is_null() const;
+  bool is_null() const { return bind_state_.get() == NULL; }
 
   // Returns the Callback into an uninitialized state.
   void Reset();