Safety check fix: add |FEATURE_UNAVAILABLE| to enums.xml

The SafetyCheck::PasswordStatus |FEATURE_UNAVAILABLE| value was listed
in the C++ enum, but was missing in the corresponding enum in enums.xml.
This CL fixes that.

Bug: 1015841
Change-Id: Ib1f67b876f5252f98f2ce5994b04635b775a0cbe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2547703Reviewed-by: Andrey Zaytsev <andzaytsev@google.com>
Commit-Queue: Rainhard Findling <rainhard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#829617}

tools/metrics/histograms/enums.xml

@@ -64433,6 +64433,7 @@ https://www.dmtf.org/sites/default/files/standards/documents/DSP0134_2.7.1.pdf
   <int value="5" label="SIGNED_OUT"/>
   <int value="6" label="QUOTA_LIMIT"/>
   <int value="7" label="FAILED"/>
+  <int value="8" label="FEATURE_UNAVAILABLE"/>
 </enum>
 
 <enum name="SafetyCheckSafeBrowsingStatus">