Make TokenService drop its ref to WebDataService in Shutdown() rather than dtor.

chrome/browser/signin/token_service.cc

Index: chrome/browser/signin/token_service.cc
diff --git a/chrome/browser/signin/token_service.cc b/chrome/browser/signin/token_service.cc
index 16681b92884460121eb26c854a34c358317ba533..17b17c148c2af6a2a297d28d878564b61ce48112 100644
--- a/chrome/browser/signin/token_service.cc
+++ b/chrome/browser/signin/token_service.cc
@@ -59,10 +59,14 @@ TokenService::TokenService()
 }
 
 TokenService::~TokenService() {
+}
+
+void TokenService::Shutdown() {
   if (!source_.empty()) {
     DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
     ResetCredentialsInMemory();
   }
+  web_data_service_ = NULL;
 }
 
 void TokenService::Initialize(const char* const source,