[TabManager] fixing comment at DiscardTabById function.

chrome/browser/memory/tab_manager.h

Index: chrome/browser/memory/tab_manager.h
diff --git a/chrome/browser/memory/tab_manager.h b/chrome/browser/memory/tab_manager.h
index c3f4a57f9cb4f3debb746dac4c70d01df048bdd9..dabc801cd1e2871957456bb63947274dfb2f7a68 100644
--- a/chrome/browser/memory/tab_manager.h
+++ b/chrome/browser/memory/tab_manager.h
@@ -101,8 +101,9 @@ class TabManager : public TabStripModelObserver {
   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.