Refactor DNS A/B experient, and add test of congestion time limits...

chrome/browser/net/dns_global.h

Index: chrome/browser/net/dns_global.h
===================================================================
--- chrome/browser/net/dns_global.h	(revision 19463)
+++ chrome/browser/net/dns_global.h	(working copy)
@@ -11,10 +11,13 @@
 #ifndef CHROME_BROWSER_NET_DNS_GLOBAL_H_
 #define CHROME_BROWSER_NET_DNS_GLOBAL_H_
 
-#include "chrome/browser/net/dns_master.h"
 
 #include <string>
 
+#include "base/field_trial.h"
+#include "base/scoped_ptr.h"
+#include "chrome/browser/net/dns_master.h"
+
 class PrefService;
 
 namespace net {
@@ -25,7 +28,8 @@
 
 // Initialize dns prefetching subsystem. Must be called before any other
 // functions.
-void InitDnsPrefetch(size_t max_concurrent, PrefService* user_prefs);
+void InitDnsPrefetch(TimeDelta max_queue_delay, size_t max_concurrent,
+                     PrefService* user_prefs);
 
 // Cancel pending lookup requests and don't make new ones. Does nothing
 // if dns prefetching has not been initialized (to simplify its usage).
@@ -66,19 +70,23 @@
 // Helper class to handle global init and shutdown.
 class DnsPrefetcherInit {
  public:
-  // Too many concurrent lookups negate benefits of prefetching by trashing the
-  // OS cache before all resource loading is complete.  This is the default.
+  // Too many concurrent lookups negate benefits of prefetching by trashing
+  // the OS cache before all resource loading is complete.
+  // This is the default.
   static const size_t kMaxConcurrentLookups;
 
-  DnsPrefetcherInit(size_t max_concurrent, PrefService* user_prefs) {
-    InitDnsPrefetch(max_concurrent, user_prefs);
-  }
+  // When prefetch requests are queued beyond some period of time, then the
+  // system is congested, and we need to clear all queued requests to get out
+  // of that state.  The following is the suggested default time limit.
+  static const int kMaxQueueingDelayMs;
 
-  ~DnsPrefetcherInit() {
-    FreeDnsPrefetchResources();
-  }
+  DnsPrefetcherInit(PrefService* user_prefs, PrefService* local_state);
+  ~DnsPrefetcherInit();
 
  private:
+  // Maintain a field trial instance when we do A/B testing.
+  scoped_refptr<FieldTrial> trial_;
+
   DISALLOW_COPY_AND_ASSIGN(DnsPrefetcherInit);
 };