Make WDDestroyableResult<T> derive from WDResult<T>.

components/webdata/common/web_data_results.h

Index: components/webdata/common/web_data_results.h
diff --git a/components/webdata/common/web_data_results.h b/components/webdata/common/web_data_results.h
index 6d454335ce8b5927754b33c2d999a33af23be42e..23a2175b77a8f9e9afbb5ae17aa151c57b0faa95 100644
--- a/components/webdata/common/web_data_results.h
+++ b/components/webdata/common/web_data_results.h
@@ -81,14 +81,13 @@ template <class T> class WDResult : public WDTypedResult {
   DISALLOW_COPY_AND_ASSIGN(WDResult);
 };
 
-template <class T> class WDDestroyableResult : public WDTypedResult {
+template <class T> class WDDestroyableResult : public WDResult<T> {
  public:
   WDDestroyableResult(
       WDResultType type,
       const T& v,
       const DestroyCallback& callback)
-      : WDTypedResult(type),
-        value_(v),
+      : WDResult<T>(type, v),
         callback_(callback) {
   }
 
@@ -102,13 +101,7 @@ template <class T> class WDDestroyableResult : public WDTypedResult {
     }
   }
 
-  // Return a single value result.
-  T GetValue() const {
-    return value_;
-  }
-
  private:
-  T value_;
   DestroyCallback callback_;
 
   DISALLOW_COPY_AND_ASSIGN(WDDestroyableResult);