Place SessionRestore/MemoryPressure interactions behind a field trial on Mac/Win platforms.

chrome/browser/sessions/tab_loader.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/sessions/tab_loader.h"
 
 #include <algorithm>
 #include <string>
 
+#include "base/metrics/field_trial.h"
 #include "base/metrics/histogram.h"
 #include "base/strings/stringprintf.h"
 #include "chrome/browser/ui/browser.h"
 #include "chrome/browser/ui/browser_finder.h"
 #include "chrome/browser/ui/tabs/tab_strip_model.h"
 #include "content/public/browser/navigation_controller.h"
 #include "content/public/browser/notification_service.h"
 #include "content/public/browser/notification_types.h"
 #include "content/public/browser/render_widget_host.h"
 #include "content/public/browser/render_widget_host_view.h"
@@ skipping 156 matching lines @@
 }
 
 void TabLoader::HandleTabClosedOrLoaded(NavigationController* controller) {
   RemoveTab(controller);
   if (delegate_ && loading_enabled_)
     LoadNextTab();
 }
 
 void TabLoader::OnMemoryPressure(
     base::MemoryPressureListener::MemoryPressureLevel memory_pressure_level) {
+  // On Windows and Mac this mechanism is only experimentally enabled.
+#if defined(OS_WIN) || (defined(OS_MACOSX) && !defined(OS_IOS))

[sky, 2015/05/08 17:37:24]

The problem with memory pressure or network disconnects is it means all the time
related histograms are completely wrong. By that I mean if we get memory
pressure during session restore we stop loading subsequent tabs. That
effectively means the histograms are not correct. That needs to be fixed before
we turn this on.

+  // If memory pressure integration isn't explicitly enabled then ignore these
+  // calls.
+  std::string react_to_memory_pressure = variations::GetVariationParamValue(
+      "IntelligentSessionRestore", "ReactToMemoryPressure");
+  if (react_to_memory_pressure != "true")
+    return;
+#endif
+
   // When receiving a resource pressure level warning, we stop pre-loading more
   // tabs since we are running in danger of loading more tabs by throwing out
   // old ones.
   if (tabs_to_load_.empty())
     return;
   // Stop the timer and suppress any tab loads while we clean the list.
   SetTabLoadingEnabled(false);
   while (!tabs_to_load_.empty()) {
     NavigationController* controller = tabs_to_load_.front();
     tabs_to_load_.pop_front();
     RemoveTab(controller);
   }
   // By calling |LoadNextTab| explicitly, we make sure that the
   // |NOTIFICATION_SESSION_RESTORE_DONE| event gets sent.
   LoadNextTab();
 }
 
 // static
 TabLoader* TabLoader::shared_tab_loader_ = nullptr;