Include ukm.xml in metrics-metadata archive.

Follow up to https://chromium-review.googlesource.com/c/chromium/src/+/978942
which added it to the build, but did not include the result in the archive.

Change-Id: I0b5d1168397cace23c56206bd59f571e89111bfa
Reviewed-on: https://chromium-review.googlesource.com/996469Reviewed-by: Michael Moss <mmoss@chromium.org>
Commit-Queue: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550402}

chrome/tools/build/win/FILES.cfg

@@ -785,6 +785,12 @@ FILES = [
     'archive': 'metrics-metadata.zip',
     'optional': ['dev', 'official'],
   },
+  {
+    'filename': 'ukm.xml',
+    'buildtype': ['dev', 'official'],
+    'archive': 'metrics-metadata.zip',
+    'optional': ['dev', 'official'],
+  },
   # MEI Preload files:
   {
     'filename': 'MEIPreload/manifest.json',