[Multipaste] return if CachedImageModel not found

It should exist, and we assume that it exists.

I'm not sure how it would not exist, but return in this case to
avoid a crash.

Bug: 1138665
Change-Id: I7a667f4e3744f7f5bb580139962b92860f763d5d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2546853Reviewed-by: Andrew Xu <andrewxu@chromium.org>
Commit-Queue: Alex Newcomer <newcomer@chromium.org>
Auto-Submit: Alex Newcomer <newcomer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828924}

ash/clipboard/clipboard_history_resource_manager.cc

@@ -277,6 +277,8 @@ void ClipboardHistoryResourceManager::OnClipboardHistoryItemRemoved(
       cached_image_models_, FindCachedImageModelForItem(item));
 
   DCHECK(cached_image_model != cached_image_models_.end());
+  if (cached_image_model == cached_image_models_.end())
+    return;
 
   // Update usages.
   base::Erase(cached_image_model->clipboard_history_item_ids, item.id());