Add a virtual destructor to WebCredentialManagerClient.

A class with virtual methods should have a virtual destructor.

BUG=697810

Review-Url: https://codereview.chromium.org/2858683005
Cr-Commit-Position: refs/heads/master@{#469001}

content/shell/test_runner/mock_credential_manager_client.h

@@ -22,7 +22,7 @@ namespace test_runner {
 class MockCredentialManagerClient : public blink::WebCredentialManagerClient {
  public:
   MockCredentialManagerClient();
-  virtual ~MockCredentialManagerClient();
+  ~MockCredentialManagerClient() override;
 
   // We take ownership of the |credential|.
   void SetResponse(blink::WebCredential* credential);

third_party/WebKit/public/platform/WebCredentialManagerClient.h

@@ -36,6 +36,9 @@ class WebCredentialManagerClient {
                            bool include_passwords,
                            const WebVector<WebURL>& federations,
                            RequestCallbacks*) {}
+
+ protected:
+  virtual ~WebCredentialManagerClient() {}
 };
 
 }  // namespace blink