Clean up histograms.xml formatting

tools/metrics/common/pretty_print_xml.py

Index: tools/metrics/common/pretty_print_xml.py
diff --git a/tools/metrics/common/pretty_print_xml.py b/tools/metrics/common/pretty_print_xml.py
index ea65778954f7935c8bbb1861e354ea62273d88c4..d47acede959f2106482bb8e896e7f508673f9070 100644
--- a/tools/metrics/common/pretty_print_xml.py
+++ b/tools/metrics/common/pretty_print_xml.py
@@ -66,10 +66,11 @@ def SplitParagraphs(text):
 class XmlStyle(object):
   """A class that stores all style specification for an output xml file."""
 
-  def __init__(self, attribute_order, tags_that_have_extra_newline,
-               tags_that_dont_indent, tags_that_allow_single_line,
-               tags_alphabetization_rules):
+  def __init__(self, attribute_order, required_attributes,
+               tags_that_have_extra_newline, tags_that_dont_indent,
+               tags_that_allow_single_line, tags_alphabetization_rules):
     self.attribute_order = attribute_order
+    self.required_attributes = required_attributes
     self.tags_that_have_extra_newline = tags_that_have_extra_newline
     self.tags_that_dont_indent = tags_that_dont_indent
     self.tags_that_allow_single_line = tags_that_allow_single_line
@@ -199,8 +200,22 @@ class XmlStyle(object):
       if not node.childNodes:
         closing_chars = 2
 
-      # Pretty-print the attributes.
       attributes = node.attributes.keys()
+      required_attributes = [attribute for attribute in self.required_attributes
+                             if attribute in self.attribute_order[node.tagName]]
+      missing_attributes = [attribute for attribute in required_attributes
+                            if attribute not in attributes]
+
+      for attribute in missing_attributes:
+        logging.error(
+            'Missing attribute "%s" in tag "%s"', attribute, node.tagName)
+      if missing_attributes:
+        raise Error(
+            'Missing attributes {0} in tag "{1}"'.format(
+                ', '.join('"{0}"'.format(a) for a in missing_attributes),

[Alexei Svitkine (slow), 2017/05/19 20:05:28]

Nit: I'd extract this inner statement into a var above so that this statement is
a bit easier to parse.

+                node.tagName))
+
+      # Pretty-print the attributes.
       if attributes:
         # Reorder the attributes.
         unrecognized_attributes = (