[iOS] Updating SSO identity metrics labels

Updating Signin.SSOIdentityListRequest metrics. The metrics should
record where the identity list come from when being requested.
This changes don't change the values already recorded.

The goal is to prepare to add a third value later:
"SSO identity list returned from stale cache"

Related to:
 + crrev.com/i/3151963

Metrics introduced with:
 + crrev.com/c/2201720
 + crrev.com/i/2995955

Change-Id: Idc3264e2864c2a1ea98cac41f61fef0c69f74399
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2283744
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Reviewed-by: Nohemi Fernandez <fernandex@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792697}

tools/metrics/histograms/enums.xml

@@ -64707,8 +64707,12 @@ https://www.dmtf.org/sites/default/files/standards/documents/DSP0134_2.7.1.pdf
 </enum>
 
 <enum name="SigninSSOIdentityListRequestCacheState">
-  <int value="0" label="SSO Identity list is stale"/>
-  <int value="1" label="SSO Identity list is valid"/>
+  <int value="0"
+      label="SSO identity cache is stale, the identity list is synchronously
+             fetched."/>
+  <int value="1"
+      label="SSO identity cache is valid, the identity list from the cache is
+             returned."/>
 </enum>
 
 <enum name="SigninSSOWKWebViewGetAllCookiesRequest">