[TabManager] fixing comment at DiscardTabById function.

chrome/browser/memory/tab_manager.h

 // Copyright (c) 2012 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 CHROME_BROWSER_MEMORY_TAB_MANAGER_H_
 #define CHROME_BROWSER_MEMORY_TAB_MANAGER_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 83 matching lines @@
   // Goes through a list of checks to see if a tab is allowed to be discarded by
   // the automatic tab discarding mechanism. Note that this is not used when
   // discarding a particular tab from about:discards.
   bool CanDiscardTab(int64_t target_web_contents_id) const;
 
   // Discards a tab to free the memory occupied by its renderer. The tab still
   // exists in the tab-strip; clicking on it will reload it.
   void DiscardTab();
 
   // Discards a tab with the given unique ID. The tab still exists in the
-  // tab-strip; clicking on it will reload it. Returns true if it successfully
-  // found a tab and discarded it.
+  // tab-strip; clicking on it will reload it. Returns null if the tab cannot
+  // be found or cannot be discarded. Otherwise returns the old web_contents
+  // that got discarded. This value is mostly useful during testing.
   content::WebContents* DiscardTabById(int64_t target_web_contents_id);
 
   // Log memory statistics for the running processes, then discards a tab.
   // Tab discard happens sometime later, as collecting the statistics touches
   // multiple threads and takes time.
   void LogMemoryAndDiscardTab();
 
   // Log memory statistics for the running processes, then call the callback.
   void LogMemory(const std::string& title, const base::Closure& callback);
 
@@ skipping 214 matching lines @@
 
   // Weak pointer factory used for posting delayed tasks to task_runner_.
   base::WeakPtrFactory<TabManager> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(TabManager);
 };
 
 }  // namespace memory
 
 #endif  // CHROME_BROWSER_MEMORY_TAB_MANAGER_H_