Created NavigationManager::CopyStateFromAndPrune().

ios/web/public/navigation_manager.h

 // Copyright 2013 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.
 
 #ifndef IOS_WEB_PUBLIC_NAVIGATION_MANAGER_H_
 #define IOS_WEB_PUBLIC_NAVIGATION_MANAGER_H_
 
 #include <stddef.h>
 
 #import "base/mac/scoped_nsobject.h"
@@ skipping 148 matching lines @@
   // TODO(crbug.com/700571): implement the logic for when |reload_type| is
   // ORIGINAL_REQUEST_URL.
   // TODO(crbug.com/700958): implement the logic for |check_for_repost|.
   virtual void Reload(ReloadType reload_type, bool check_for_repost) = 0;
 
   // Returns a list of all non-redirected NavigationItems whose index precedes
   // or follows the current index.
   virtual NavigationItemList GetBackwardItems() const = 0;
   virtual NavigationItemList GetForwardItems() const = 0;
 
+  // Removes all items from this except the last committed item, and inserts
+  // copies of all items from |source| at the beginning of the session history.
+  //
+  // For example:
+  // source: A B *C* D
+  // this:   E F *G*
+  // result: A B C *G*
+  //
+  // If there is a pending item after *G* in |this|, it is also preserved.
+  // This ignores any pending or transient entries in |source|.  This will be a
+  // no-op if called while CanPruneAllButLastCommittedItem() is false.
+  virtual void CopyStateFromAndPrune(const NavigationManager* source) = 0;
+
+  // Whether the NavigationManager can prune all but the last committed item.
+  // This is true when all the following conditions are met:
+  // - There is a last committed NavigationItem.
+  // - There is no pending history navigation.
+  // - There is no transient NavigationItem.
+  virtual bool CanPruneAllButLastCommittedItem() const = 0;

[Eugene But (OOO till 7-30), 2017/03/15 19:54:51]

Please add tests for this new method.

[kkhorimoto, 2017/03/15 21:04:38]

The CRWSessionController implementation is already tested by its unittests. 
I'll move the tests and the implementations to NavigationManager at the same
time.

+
   // Forces the pending item to be loaded using desktop user agent. Note that
   // the pending item may or may not already exist.
   // TODO(crbug.com/692303): Remove this when overriding the user agent doesn't
   // create a new NavigationItem.
   virtual void OverrideDesktopUserAgentForNextPendingItem() = 0;
 };
 
 }  // namespace web
 
 #endif  // IOS_WEB_PUBLIC_NAVIGATION_MANAGER_H_