[Passwords] Use enum instead of units for Change Password Histograms

This change fixes up the GstaticFetchHttpResponseCode and
GstaticFetchResult histograms in PasswordManager.WellKnownChangePassword
to use `enum` instead of `units`.

R=chromium-metrics-reviews@google.com

Bug: 927473
Change-Id: I23e8838e065dd21bc433747f64a54d2a85da6f47
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2485229
Auto-Submit: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Weilun Shi <sweilun@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821989}

tools/metrics/histograms/histograms_xml/password/histograms.xml

@@ -2225,7 +2225,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="PasswordManager.WellKnownChangePassword.GstaticFetchErrorCode"
-    units="NetErrorCode" expires_after="2021-01-31">
+    enum="NetErrorCodes" expires_after="2021-01-31">
   <owner>jdoerrie@chromium.org</owner>
   <owner>vasilii@chromium.org</owner>
   <summary>
@@ -2235,7 +2235,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 
 <histogram
     name="PasswordManager.WellKnownChangePassword.GstaticFetchHttpResponseCode"
-    units="HttpResponseCode" expires_after="2021-01-31">
+    enum="HttpResponseCode" expires_after="2021-01-31">
   <owner>jdoerrie@chromium.org</owner>
   <owner>vasilii@chromium.org</owner>
   <summary>