Fixed leak on LRU snapshot cache implementation.

ios/chrome/browser/snapshots/snapshot_cache.mm

Index: ios/chrome/browser/snapshots/snapshot_cache.mm
diff --git a/ios/chrome/browser/snapshots/snapshot_cache.mm b/ios/chrome/browser/snapshots/snapshot_cache.mm
index b76ba6ec4889a46a04b4918c50947aa44e51b81c..05622576f634a4d0ff93bd8257d4207983a80476 100644
--- a/ios/chrome/browser/snapshots/snapshot_cache.mm
+++ b/ios/chrome/browser/snapshots/snapshot_cache.mm
@@ -389,8 +389,8 @@ void ConvertAndSaveGreyImage(
 - (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_)
@@ -403,9 +403,10 @@ void ConvertAndSaveGreyImage(
   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;
   }
 }