Oilpan: add missing leftmost trace()s for GC mixins.

Source/modules/credentialmanager/CredentialManagerClient.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CredentialManagerClient_h
 #define CredentialManagerClient_h
 
 #include "platform/Supplementable.h"
 #include "public/platform/WebCredentialManagerClient.h"
 #include "public/platform/WebVector.h"
 
 namespace blink {
 
 class ExecutionContext;
 class Page;
 class WebCredential;
 class WebURL;
 
 // CredentialManagerClient lives as a supplement to Page, and wraps the embedder-provided
 // WebCredentialManagerClient's methods to make them visible to the bindings code.
 class CredentialManagerClient final : public NoBaseWillBeGarbageCollectedFinalized<CredentialManagerClient>, public WillBeHeapSupplement<Page> {
     WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(CredentialManagerClient);
 public:
     explicit CredentialManagerClient(WebCredentialManagerClient*);
     virtual ~CredentialManagerClient();
+    virtual void trace(Visitor*) override;
 
     static const char* supplementName();
     static CredentialManagerClient* from(Page*);
     static CredentialManagerClient* from(ExecutionContext*);
 
     // Ownership of the callback is transferred to the callee for each of
     // the following methods.
     virtual void dispatchFailedSignIn(const WebCredential&, WebCredentialManagerClient::NotificationCallbacks*);
     virtual void dispatchSignedIn(const WebCredential&, WebCredentialManagerClient::NotificationCallbacks*);
     virtual void dispatchSignedOut(WebCredentialManagerClient::NotificationCallbacks*);
     virtual void dispatchRequest(bool zeroClickOnly, const WebVector<WebURL>& federations, WebCredentialManagerClient::RequestCallbacks*);
 
 private:
     WebCredentialManagerClient* m_client;
 };
 
 void provideCredentialManagerClientTo(Page&, CredentialManagerClient*);
 
 } // namespace blink
 
 #endif // CredentialManagerClient_h