Uncommenting extract_owners in merge_xml

This was accidentally commented in the split xml's cl because we don't
want to extract OWNERS while splitting them.

Change-Id: Ibe3a566d46a64c1a41ae5eb9c7a1eddcd008ca83
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2450733
Commit-Queue: Steven Holte <holte@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Auto-Submit: Weilun Shi <sweilun@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813851}

tools/metrics/histograms/merge_xml.py

@@ -128,8 +128,8 @@ def MakeNodeWithChildren(doc, tag, children):
   """
   node = doc.createElement(tag)
   for child in children:
-    # if child.tagName == 'histograms':
-    #   expand_owners.ExpandHistogramsOWNERS(child)
+    if child.tagName == 'histograms':
+      expand_owners.ExpandHistogramsOWNERS(child)
     node.appendChild(child)
   return node