Add HostCachePersistenceManager for Cronet

net/dns/host_cache.cc

Index: net/dns/host_cache.cc
diff --git a/net/dns/host_cache.cc b/net/dns/host_cache.cc
index 19a2192698fb8f85b70ad92212c4e4acc0719696..380bae8195672d93906371a7e3b0174539af221d 100644
--- a/net/dns/host_cache.cc
+++ b/net/dns/host_cache.cc
@@ -248,6 +248,13 @@ void HostCache::OnNetworkChange() {
   ++network_changes_;
 }
 
+void HostCache::set_persistence_delegate(PersistenceDelegate* delegate) {
+  // A PersistenceDelegate shouldn't be added if there already was one, and
+  // shouldn't be removed (by setting to nullptr) if it wasn't previously there.
+  DCHECK_NE(delegate == nullptr, delegate_ == nullptr);
+  delegate_ = delegate;
+}
+
 void HostCache::clear() {
   DCHECK_CALLED_ON_VALID_THREAD(thread_checker_);
   RecordEraseAll(ERASE_CLEAR, base::TimeTicks::Now());