Fix build breakage caused by Python file rename.

This was broken by:
https://chromium.googlesource.com/chromium/src/+/5c5ce638e36966b872f26e2df86116388e06fd51

TBR=holte@chromium.org

Bug: 892444
Change-Id: I8e379fe5d1a76ce01e21cc96c9b4d953dfad40d9
Reviewed-on: https://chromium-review.googlesource.com/c/1263237
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596933}

tools/metrics/ukm/gen_builders.py

@@ -13,7 +13,7 @@ entries and metrics.
 import argparse
 import sys
 
-import model
+import ukm_model
 import builders_template
 import decode_template
 
@@ -24,7 +24,7 @@ parser.add_argument('--output', help='Path to generated files.')
 
 def main(argv):
   args = parser.parse_args()
-  data = model.UKM_XML_TYPE.Parse(open(args.input).read())
+  data = ukm_model.UKM_XML_TYPE.Parse(open(args.input).read())
   relpath = 'services/metrics/public/cpp/'
   builders_template.WriteFiles(args.output, relpath, data)
   decode_template.WriteFiles(args.output, relpath, data)