[Session restore] Expose lazy mode and fix browser tests.

chrome/browser/sessions/session_restore.cc

Index: chrome/browser/sessions/session_restore.cc
diff --git a/chrome/browser/sessions/session_restore.cc b/chrome/browser/sessions/session_restore.cc
index 82d6432b38bc273c043928e2093f387a7f7fa0f1..b1f27335cceafe75034aa52f6562f9a1c5d4e1d3 100644
--- a/chrome/browser/sessions/session_restore.cc
+++ b/chrome/browser/sessions/session_restore.cc
@@ -16,6 +16,7 @@
 #include "base/debug/alias.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/memory/scoped_vector.h"
+#include "base/metrics/field_trial.h"
 #include "base/metrics/histogram.h"
 #include "base/run_loop.h"
 #include "base/stl_util.h"
@@ -298,7 +299,9 @@ class SessionRestoreImpl : public content::NotificationObserver {
 
     if (succeeded) {
       // Start Loading tabs.
-      SessionRestoreDelegate::RestoreTabs(contents_created, restore_started_);
+      bool active_only = SessionRestore::WillLoadActiveTabsOnly();
+      SessionRestoreDelegate::RestoreTabs(contents_created, restore_started_,
+                                          active_only);
     }
 
     if (!synchronous_) {
@@ -818,5 +821,14 @@ SessionRestore::CallbackSubscription
 }
 
 // static
+bool SessionRestore::WillLoadActiveTabsOnly() {
+  base::FieldTrial* trial =
+      base::FieldTrialList::Find("SessionRestoreBackgroundLoading");
+  if (!trial || trial->group_name() == "Restore")
+    return false;
+  return true;
+}
+
+// static
 base::CallbackList<void(int)>*
     SessionRestore::on_session_restored_callbacks_ = nullptr;