Fix WebStateListObserver::WebStateDetachedAt() parameter lifetime.

ios/chrome/browser/tabs/tab_model.mm

 // Copyright 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.
 
 #import "ios/chrome/browser/tabs/tab_model.h"
 
 #include <cstdint>
 #include <utility>
 #include <vector>
 
@@ skipping 575 matching lines @@
 
   int index = _webStateList.GetIndexOfWebState(oldTab.webState);
   DCHECK_NE(index, WebStateList::kInvalidIndex);
   DCHECK_GE(index, 0);
 
   base::scoped_nsobject<Tab> tabSaver([oldTab retain]);
   [_tabRetainer removeObject:oldTab];
   [_tabRetainer addObject:newTab];
   [newTab setParentTabModel:self];
 
-  _webStateList.ReplaceWebStateAt(index, newTab.webState,
-                                  GetOpenerForTab(self, newTab).webState);
+  // The WebState is owned by the associated Tab, so it is safe to ignore
+  // the result and won't cause a memory leak. Once the ownershipd is moved
+  // to WebStateList, this function will return a std::unique_ptr<> and the
+  // object destroyed as expected, so it will fine to ignore the result then
+  // too. See http://crbug.com/546222 for progress of changing the ownership
+  // of the WebStates.
+  ignore_result(_webStateList.ReplaceWebStateAt(
+      index, newTab.webState, GetOpenerForTab(self, newTab).webState));
 
   if (self.currentTab == oldTab)
     [self changeSelectedTabFrom:nil to:newTab persistState:NO];
 
   [oldTab setParentTabModel:nil];
   [oldTab close];
 }
 
 - (void)closeTabAtIndex:(NSUInteger)index {
   DCHECK(index < self.count);
@@ skipping 143 matching lines @@
   // If closing the current tab, clear |_currentTab| before sending any
   // notification. This avoids various parts of the code getting confused
   // when the current tab isn't in the tab model.
   Tab* savedCurrentTab = _currentTab;
   if (closedTab == _currentTab)
     _currentTab.reset(nil);
 
   DCHECK([_tabRetainer containsObject:closedTab]);
   [_tabRetainer removeObject:closedTab];
 
-  _webStateList.DetachWebStateAt(closedTabIndex);
+  // The WebState is owned by the associated Tab, so it is safe to ignore
+  // the result and won't cause a memory leak. Once the ownershipd is moved
+  // to WebStateList, this function will return a std::unique_ptr<> and the
+  // object destroyed as expected, so it will fine to ignore the result then
+  // too. See http://crbug.com/546222 for progress of changing the ownership
+  // of the WebStates.
+  ignore_result(_webStateList.DetachWebStateAt(closedTabIndex));
 
   // Current tab has closed, update the selected tab and swap in its
   // contents. There is nothing to do if a non-selected tab is closed as
   // the selection isn't index-based, therefore it hasn't changed.
   // -changeSelectedTabFrom: will persist the state change, so only do it
   // if the selection isn't changing.
   if (closedTab == savedCurrentTab) {
     [self changeSelectedTabFrom:closedTab to:newSelection persistState:NO];
   } else {
     [self saveSessionImmediately:NO];
@@ skipping 360 matching lines @@
   web::NavigationManager::WebLoadParams params(URL);
   params.referrer = referrer;
   params.transition_type = ui::PAGE_TRANSITION_TYPED;
   [[tab webController] loadWithParams:params];
   [tab webController].webUsageEnabled = webUsageEnabled_;
   [self insertTab:tab atIndex:index opener:parentTab];
   return tab;
 }
 
 @end