Don't move raw pointers in HTMLCollection.h

Moving raw pointers does the same job as copying, but is more verbose and also
more confusing: e.g., is the supposed pointer meant to be a smart one?

This instance was flagged by the tool from
https://codereview.chromium.org/2919243002/.

BUG=729393

Review-Url: https://codereview.chromium.org/2927233003
Cr-Commit-Position: refs/heads/master@{#478589}

third_party/WebKit/Source/core/html/HTMLCollection.h

@@ -180,7 +180,7 @@ class CORE_EXPORT HTMLCollection
     // Do not repeat registration for the same invalidation type.
     if (InvalidationType() != kInvalidateOnIdNameAttrChange)
       GetDocument().RegisterNodeListWithIdNameCache(this);
-    named_item_cache_ = std::move(cache);
+    named_item_cache_ = cache;
   }
 
   NamedItemCache& GetNamedItemCache() const {