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

chrome/browser/sessions/session_restore.h

Index: chrome/browser/sessions/session_restore.h
diff --git a/chrome/browser/sessions/session_restore.h b/chrome/browser/sessions/session_restore.h
index e6cb7819e514e7319d77cb150a8902ad28df3234..3d675a20a6d08458f1052f794edf4f212983a912 100644
--- a/chrome/browser/sessions/session_restore.h
+++ b/chrome/browser/sessions/session_restore.h
@@ -98,6 +98,11 @@ class SessionRestore {
   static CallbackSubscription RegisterOnSessionRestoredCallback(
       const base::Callback<void(int)>& callback);
 
+  // Returns true if only active tabs are to be loaded during session restore
+  // (lazy load). Otherwise returns false if all tabs will be loaded when
+  // restored.
+  static bool WillLoadActiveTabsOnly();
+
  private:
   SessionRestore();