Add presubmit to force existence of expires_after histogram attribute.

A change to histograms.xml was needed to validate the presubmit so
<obsolete> messages had "deprecated" changed to "removed".
"Deprecated" means it still exists (but is in disfavor) when in fact
the histograms have been completely removed.

Removal of an expires_after triggered the presubmit error but the file
was otherwise okay.

Bug: 1030096
Change-Id: I07422aabc6b1fdcf9e65ce1abc792239c865e64c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2052623
Commit-Queue: Brian White <bcwhite@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#741432}

tools/metrics/histograms/extract_histograms.py

@@ -433,6 +433,12 @@ def _ExtractHistogramsFromXmlTree(tree, enums):
             ' milestone format (M*), or "never": found %s.', name,
             EXPIRY_DATE_PATTERN, expiry_str)
         have_errors = True
+    else:
+      logging.error(
+          'Your histogram must have an expiry date. If you are marking a '
+          'histogram as obsolete, please set the expiry date to the current '
+          'date.')
+      have_errors = True
 
     # Find <owner> tag.
     owners, has_owner = _ExtractOwners(histogram)