Fixed leak on LRU snapshot cache implementation.

ios/chrome/browser/snapshots/snapshot_cache.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/snapshots/snapshot_cache.h"
 
 #import <UIKit/UIKit.h>
 
 #include "base/critical_closure.h"
 #include "base/files/file_enumerator.h"
@@ skipping 371 matching lines @@
     backgroundingColorImage_.reset([[lruCache_ objectForKey:sessionID] retain]);
   } else {
     backgroundingColorImage_.reset(
         [[imageDictionary_ objectForKey:sessionID] retain]);
   }
 }
 
 - (void)handleLowMemory {
   DCHECK(!IsIPadIdiom());
   DCHECK_CURRENTLY_ON_WEB_THREAD(web::WebThread::UI);
-  NSMutableDictionary* dictionary =
-      [[NSMutableDictionary alloc] initWithCapacity:2];
+  base::scoped_nsobject<NSMutableDictionary> dictionary(
+      [[NSMutableDictionary alloc] initWithCapacity:2]);
   for (NSString* sessionID in pinnedIDs_) {
     UIImage* image = nil;
     if (lruCache_)
       image = [lruCache_ objectForKey:sessionID];
     else
       image = [imageDictionary_ objectForKey:sessionID];
     if (image)
       [dictionary setObject:image forKey:sessionID];
   }
   if (lruCache_) {
     [lruCache_ removeAllObjects];
     for (NSString* sessionID in pinnedIDs_)
-      [lruCache_ setObject:dictionary[sessionID] forKey:sessionID];
+      [lruCache_ setObject:[dictionary objectForKey:sessionID]
+                    forKey:sessionID];
   } else {
-    imageDictionary_.reset(dictionary);
+    imageDictionary_ = dictionary;
   }
 }
 
 - (void)handleEnterBackground {
   DCHECK(!IsIPadIdiom());
   DCHECK_CURRENTLY_ON_WEB_THREAD(web::WebThread::UI);
   [imageDictionary_ removeAllObjects];
   [lruCache_ removeAllObjects];
 }
 
@@ skipping 156 matching lines @@
 }
 - (BOOL)hasGreyImageInMemory:(NSString*)sessionID {
   return [greyImageDictionary_ objectForKey:sessionID] != nil;
 }
 
 - (NSUInteger)lruCacheMaxSize {
   return [lruCache_ maxCacheSize];
 }
 
 @end