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

This fixes an issue found by -fsanitize=cfi-derived-cast caused by code that
casts a WDResult<T> pointer to WDDestroyableResult<T>.

BUG=457523
R=caitkp@chromium.org

Review URL: https://codereview.chromium.org/1021043002

Cr-Commit-Position: refs/heads/master@{#321644}

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);