RFC: use some in-memory state in SimpleCache to quickly cache-miss some CantConditionalize cases

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 51c0db70256cbd31505ca20ffba6486ab4c36aad..b2398874b56708f70e45877c01784a3b96008449 100644
--- a/net/http/http_cache_transaction.h
+++ b/net/http/http_cache_transaction.h
@@ -345,6 +345,11 @@ class HttpCache::Transaction : public HttpTransaction {
   // copy is valid).  Returns true if able to make the request conditional.
   bool ConditionalizeRequest();
 
+  // Determins if saved response permits conditionalization, and extracts
+  // etag/last-modified values. Only depends on response_.headers.
+  bool ResponseConditionalizable(std::string* etag_value,
+                                 std::string* last_modified_value);
+
   // Makes sure that a 206 response is expected.  Returns true on success.
   // On success, handling_206_ will be set to true if we are processing a
   // partial entry.