[Metrics] Fix histogram boundary error string

The correct value is kMaxValue, not kLast.

Change-Id: I8abbf39f86818cf2f6b185ee20c27c2396d05c0b
Reviewed-on: https://chromium-review.googlesource.com/1032992
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555344}

base/metrics/histogram_macros_internal.h

@@ -27,8 +27,9 @@ namespace internal {
 template <typename Enum, typename SFINAE = void>
 struct EnumSizeTraits {
   static constexpr Enum Count() {
-    static_assert(sizeof(Enum) == 0,
-                  "enumerator must define kLast enumerator to use this macro!");
+    static_assert(
+        sizeof(Enum) == 0,
+        "enumerator must define kMaxValue enumerator to use this macro!");
     return Enum();
   }
 };