Fix a typo error.

Store expiry date as string.

Bug: 744672
Change-Id: I83f9548ebe6a3a1e004d48476a5eed020d37b817
Reviewed-on: https://chromium-review.googlesource.com/578438
Commit-Queue: Ira Burak <iburak@google.com>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488668}

tools/metrics/histograms/extract_histograms.py

@@ -301,7 +301,9 @@ def _ExtractHistogramsFromXmlTree(tree, enums):
             ' found %s.',
             name, EXPIRY_DATE_PATTERN, expiry_date_str)
         have_errors = True
-      histograms[expiry_date] = expiry_date
+      else:
+        histogram_entry['expiry_date'] = expiry_date.strftime(
+            EXPIRY_DATE_PATTERN)
 
     # Find <owner> tag.
     owners = _ExtractOwners(histogram)