Support 'never' as expiry date.

Bug: 923083
Change-Id: I25136857b84832ac896e76f32c182af8eb190e15
Reviewed-on: https://chromium-review.googlesource.com/c/1418517Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625353}

tools/metrics/histograms/extract_histograms.py

@@ -303,14 +303,14 @@ def _ExtractHistogramsFromXmlTree(tree, enums):
     # Handle expiry attribute.
     if histogram.hasAttribute('expires_after'):
       expiry_str = histogram.getAttribute('expires_after')
-      if _ValidateMilestoneString(expiry_str) or _ValidateDateString(
-          expiry_str):
+      if (expiry_str == "never" or _ValidateMilestoneString(expiry_str) or
+          _ValidateDateString(expiry_str)):
         histogram_entry['expires_after'] = expiry_str
       else:
         logging.error(
-            'Expiry of histogram %s does not match expected date format: "%s"'
-            ' or milestone format: M* found %s.', name, EXPIRY_DATE_PATTERN,
-            expiry_str)
+            'Expiry of histogram %s does not match expected date format ("%s"),'
+            ' milestone format (M*), or "never": found %s.', name,
+            EXPIRY_DATE_PATTERN, expiry_str)
         have_errors = True
 
     # Find <owner> tag.

tools/metrics/histograms/generate_expired_histograms_array.py

@@ -67,6 +67,8 @@ def _GetExpiredHistograms(histograms, base_date, current_milestone):
     if "obsolete" in content or "expires_after" not in content:
       continue
     expiry_str = content["expires_after"]
+    if expiry_str == "never":
+      continue
 
     match = _MILESTONE_EXPIRY_RE.search(expiry_str)
     if match: