Make histogram_suffixes separators and enum labels required.

tools/metrics/actions/print_style.py

Index: tools/metrics/actions/print_style.py
diff --git a/tools/metrics/actions/print_style.py b/tools/metrics/actions/print_style.py
index 39deb2300e3984b679a2d74f3f8d78cfc2e4cee4..817b148ebb8fd5f971fb9e708f173bc528ea4c5c 100644
--- a/tools/metrics/actions/print_style.py
+++ b/tools/metrics/actions/print_style.py
@@ -27,6 +27,13 @@ ATTRIBUTE_ORDER = {
     'with-suffix': ['name'],
 }
 
+# Attribute names that must be explicitly specified on nodes that support them.
+REQUIRED_ATTRIBUTES = [
+    'label',
+    'name',
+    'separator',
+]
+
 # Tag names for top-level nodes whose children we don't want to indent.
 TAGS_THAT_DONT_INDENT = [
     'actions',
@@ -59,6 +66,7 @@ TAGS_ALPHABETIZATION_RULES = {
 def GetPrintStyle():
   """Returns an XmlStyle object for pretty printing actions."""
   return pretty_print_xml.XmlStyle(ATTRIBUTE_ORDER,
+                                   REQUIRED_ATTRIBUTES,
                                    TAGS_THAT_HAVE_EXTRA_NEWLINE,
                                    TAGS_THAT_DONT_INDENT,
                                    TAGS_THAT_ALLOW_SINGLE_LINE,