Use UpdateDistributionForFlatTreeTraversal in selector_query

This is a follow-up CL for http://crrev.com/c/1025155.
SelectorQuery updates distribution only for traversing a flat tree.

Bug: 776656
Change-Id: I07312acc73abdc593a7ef81f04761bacc3d978d0
Reviewed-on: https://chromium-review.googlesource.com/1029560Reviewed-by: Takayoshi Kochi <kochi@chromium.org>
Commit-Queue: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553938}

third_party/blink/renderer/core/css/selector_query.cc

@@ -111,7 +111,7 @@ inline bool SelectorMatches(const CSSSelector& selector,
 bool SelectorQuery::Matches(Element& target_element) const {
   QUERY_STATS_RESET();
   if (needs_updated_distribution_)
-    target_element.UpdateDistributionForUnknownReasons();
+    target_element.UpdateDistributionForFlatTreeTraversal();
   return SelectorListMatches(target_element, target_element);
 }
 
@@ -120,7 +120,7 @@ Element* SelectorQuery::Closest(Element& target_element) const {
   if (selectors_.IsEmpty())
     return nullptr;
   if (needs_updated_distribution_)
-    target_element.UpdateDistributionForUnknownReasons();
+    target_element.UpdateDistributionForFlatTreeTraversal();
 
   for (Element* current_element = &target_element; current_element;
        current_element = current_element->parentElement()) {