Add GetExpirationTimes() to HttpResponseHeader.

net/http/http_cache_transaction.h

Index: net/http/http_cache_transaction.h
diff --git a/net/http/http_cache_transaction.h b/net/http/http_cache_transaction.h
index 0c9b11253543ac44c65986bdd743c45870c03972..1572ae73c20eda4270e8508ef4c24330b6ed45ef 100644
--- a/net/http/http_cache_transaction.h
+++ b/net/http/http_cache_transaction.h
@@ -46,6 +46,13 @@ class SSLPrivateKey;
 // factory.
 class HttpCache::Transaction : public HttpTransaction {
  public:
+  // Used for |RequiresValidation()|.
+  enum class ValidationType {
+    NONE,          // The resource is fresh.
+    ASYNCHRONOUS,  // The resource requires async revalidation.
+    SYNCHRONOUS,   // The resource requires sync revalidation.
+  };
+
   // The transaction has the following modes, which apply to how it may access
   // its cache entry.
   //