Make flat containers stable, allow constructing from vector.

components/omnibox/browser/in_memory_url_index_types.cc

Index: components/omnibox/browser/in_memory_url_index_types.cc
diff --git a/components/omnibox/browser/in_memory_url_index_types.cc b/components/omnibox/browser/in_memory_url_index_types.cc
index 69798d19fcaf8f9710ce4272acc90b80c2e73bce..65ae6f4857f742972c23ae12c43ee47ab1579a5d 100644
--- a/components/omnibox/browser/in_memory_url_index_types.cc
+++ b/components/omnibox/browser/in_memory_url_index_types.cc
@@ -100,7 +100,8 @@ String16Set String16SetFromString16(const base::string16& cleaned_uni_string,
   for (auto& word : words)
     word = base::i18n::ToLower(word).substr(0, kMaxSignificantChars);
   return String16Set(std::make_move_iterator(words.begin()),
-                     std::make_move_iterator(words.end()));
+                     std::make_move_iterator(words.end()),
+                     base::KEEP_FIRST_OF_DUPES);
 }
 
 String16Vector String16VectorFromString16(
@@ -139,7 +140,7 @@ String16Vector String16VectorFromString16(
 }
 
 Char16Set Char16SetFromString16(const base::string16& term) {
-  return Char16Set(term.begin(), term.end());
+  return Char16Set(term.begin(), term.end(), base::KEEP_FIRST_OF_DUPES);
 }
 
 // HistoryInfoMapValue ---------------------------------------------------------