WTF::HashSet/HashMap: Clarify the restriction on integer keys.

This patch only touches comments. No code change.

Change-Id: Ibc823deb7cfe35dfed400566767e1791ac78e48b
Reviewed-on: https://chromium-review.googlesource.com/564876Reviewed-by: Taiju Tsuiki <tzik@chromium.org>
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485202}

third_party/WebKit/Source/platform/wtf/HashMap.h

@@ -39,8 +39,9 @@ struct KeyValuePairKeyExtractor {
 };
 
 // Note: empty or deleted key values are not allowed, using them may lead to
-// undefined behavior.  For pointer keys this means that null pointers are not
-// allowed unless you supply custom key traits.
+// undefined behavior. For pointer keys this means that null pointers are not
+// allowed; for integer keys 0 or -1 can't be used as a key. This restriction
+// can be lifted if you supply custom key traits.
 template <typename KeyArg,
           typename MappedArg,
           typename HashArg = typename DefaultHash<KeyArg>::Hash,

third_party/WebKit/Source/platform/wtf/HashSet.h

@@ -30,8 +30,9 @@ namespace WTF {
 struct IdentityExtractor;
 
 // Note: empty or deleted values are not allowed, using them may lead to
-// undefined behavior.  For pointer valuess this means that null pointers are
-// not allowed unless you supply custom traits.
+// undefined behavior. For pointer keys this means that null pointers are not
+// allowed; for integer keys 0 or -1 can't be used as a key. This restriction
+// can be lifted if you supply custom key traits.
 template <typename ValueArg,
           typename HashArg = typename DefaultHash<ValueArg>::Hash,
           typename TraitsArg = HashTraits<ValueArg>,