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

chrome/browser/sessions/tab_loader.cc

Index: chrome/browser/sessions/tab_loader.cc
diff --git a/chrome/browser/sessions/tab_loader.cc b/chrome/browser/sessions/tab_loader.cc
index 28b80ef4ae29c5ea9bf497e1afe7528a7eac47f4..810c6427606e98ce2953833c9ce06e3ec465f0f4 100644
--- a/chrome/browser/sessions/tab_loader.cc
+++ b/chrome/browser/sessions/tab_loader.cc
@@ -7,6 +7,7 @@
 #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"
@@ -183,6 +184,16 @@ void TabLoader::HandleTabClosedOrLoaded(NavigationController* controller) {
 
 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.