base: Add comments warning about using insert() a lot on flat_set.

base/containers/flat_set.h

Index: base/containers/flat_set.h
diff --git a/base/containers/flat_set.h b/base/containers/flat_set.h
index 21334ebe098177f1810f95846e44200846604155..ae444c2c8e21c38ebec295ee023a365b84b3e057 100644
--- a/base/containers/flat_set.h
+++ b/base/containers/flat_set.h
@@ -42,6 +42,9 @@ namespace base {
 //  * Copying and iterating.
 //  * Set operations (union/intersect etc).
 //
+// Prefer to build a new flat_set from a std::vector (or similar) instead of
+// calling insert() repeatedly, which would have O(size^2) complexity.
+//
 // TODO(dyaroshev): develop standalone benchmarks to find performance boundaries
 // for different types of sets crbug.com/682215.
 //
@@ -189,6 +192,9 @@ class flat_set {
   // Insertion of one element can take O(size). See the Notes section in the
   // class comments on why we do not currently implement range insertion.
   // Capacity of flat_set grows in an implementation-defined manner.
+  //
+  // NOTE: Prefer to build a new flat_set from a std::vector (or similar)
+  // instead of calling insert() repeatedly.
 
   std::pair<iterator, bool> insert(const value_type& val);
   std::pair<iterator, bool> insert(value_type&& val);