Add FlushStore to CookieStore interface.

net/cookies/cookie_store_test_helpers.cc

Index: net/cookies/cookie_store_test_helpers.cc
diff --git a/net/cookies/cookie_store_test_helpers.cc b/net/cookies/cookie_store_test_helpers.cc
index e75039c1484128ffbe24d2b008786bebee5aef42..b1733328e0645e115aadbb12654607236dca60ef 100644
--- a/net/cookies/cookie_store_test_helpers.cc
+++ b/net/cookies/cookie_store_test_helpers.cc
@@ -150,6 +150,10 @@ void DelayedCookieMonster::DeleteSessionCookiesAsync(const DeleteCallback&) {
   ADD_FAILURE();
 }
 
+void DelayedCookieMonster::FlushStore(const base::Closure& callback) {
+  ADD_FAILURE();
+}
+
 CookieMonster* DelayedCookieMonster::GetCookieMonster() {
   return cookie_monster_.get();
 }