[iOS] Adding metrics for chrome identity methods

Adding:
* Signin.SSOIdentityListRequest.FetchIdentitiesWithCallback.Duration
  Time needed to fetch the identity list

* Signin.SSOIdentityListRequest.RunAfterCacheIsPopulated.CacheState
  State of the identity cache before calling RunAfterCacheIsPopulated()

* Signin.SSOIdentityListRequest.RunAfterCacheIsPopulated.Duration
  Time needed for the callback to be called for
  RunAfterCacheIsPopulated()

* Signin.SSOIdentityListRequest.WaitUntilCacheIsPopulated.CacheState
  State of the identity cache before calling RunAfterCacheIsPopulated()

* Signin.SSOIdentityListRequest.WaitUntilCacheIsPopulated.Duration
  Time blocked in WaitUntilCacheIsPopulated()

Those metrics are implemented in crrev.com/i/3353162.

Design doc:
http://go/async-identityservice-api

   crrev.com/c/2489915 Adding methods in ChromeIdentityService
   crrev.com/i/3353161 Implementing in ChromeIdentityServiceImpl
=> crrev.com/c/2490110 Adding metrics
   crrev.com/i/3353162 Implementing the metrics
   crrev.com/c/2490010 Adding WaitUntilCacheIsPopulated calls
   crrev.com/i/3354983 Adding DCHECK for cache state

Bug: 897470
Change-Id: I83744b7eaec3f7eb4c2515550e6a2638e5c0ec15
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2490110
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824387}

tools/metrics/histograms/histograms_xml/signin/histograms.xml

@@ -946,6 +946,7 @@ kept as long as profile signout is possible -->
 
   <owner>jlebel@chromium.org</owner>
   <owner>fernandex@chromium.org</owner>
+  <owner>chrome-signin-team@google.com</owner>
   <summary>
     Tracks the SSO identity cache state before calling -[GCRSSOService
     identitiesWithError:] or -[GCRSSOService identitiesForDisplay]. See:
@@ -959,12 +960,77 @@ kept as long as profile signout is possible -->
 
   <owner>jlebel@chromium.org</owner>
   <owner>fernandex@chromium.org</owner>
+  <owner>chrome-signin-team@google.com</owner>
   <summary>
     Tracks the time it takes to call -[GCRSSOService identitiesWithError:] or
     -[GCRSSOService identitiesForDisplay]. See: https://crbug.com/897470.
   </summary>
 </histogram>
 
+<histogram
+    name="Signin.SSOIdentityListRequest.FetchIdentitiesWithCallback.Duration"
+    units="ms" expires_after="2021-04-19">
+  <owner>jlebel@chromium.org</owner>
+  <owner>fernandex@chromium.org</owner>
+  <owner>chrome-signin-team@google.com</owner>
+  <summary>
+    Tracks the time between -[GCRSSOService fetchIdentitiesWithCallback:] and
+    the callback is invoked. See: https://crbug.com/897470.
+  </summary>
+</histogram>
+
+<histogram
+    name="Signin.SSOIdentityListRequest.RunAfterCacheIsPopulated.CacheState"
+    enum="SigninSSOIdentityListRequestCacheState" expires_after="2021-04-19">
+  <owner>jlebel@chromium.org</owner>
+  <owner>fernandex@chromium.org</owner>
+  <owner>chrome-signin-team@google.com</owner>
+  <summary>
+    Tracks the state of the SSO identity cache before
+    ChromeIdentityServiceImpl::RunAfterCacheIsPopulated() is called. See:
+    https://crbug.com/897470.
+  </summary>
+</histogram>
+
+<histogram
+    name="Signin.SSOIdentityListRequest.RunAfterCacheIsPopulated.Duration"
+    units="ms" expires_after="2021-04-19">
+  <owner>jlebel@chromium.org</owner>
+  <owner>fernandex@chromium.org</owner>
+  <owner>chrome-signin-team@google.com</owner>
+  <summary>
+    Tracks the time between
+    ChromeIdentityServiceImpl::RunAfterCacheIsPopulated() and the callback is
+    invoked. See: https://crbug.com/897470.
+  </summary>
+</histogram>
+
+<histogram
+    name="Signin.SSOIdentityListRequest.WaitUntilCacheIsPopulated.CacheState"
+    enum="SigninSSOIdentityListRequestCacheState" expires_after="2021-04-19">
+  <owner>jlebel@chromium.org</owner>
+  <owner>fernandex@chromium.org</owner>
+  <owner>chrome-signin-team@google.com</owner>
+  <summary>
+    Tracks the state of the SSO identity cache before
+    ChromeIdentityServiceImpl::WaitUntilCacheIsPopulated() is called. See:
+    https://crbug.com/897470.
+  </summary>
+</histogram>
+
+<histogram
+    name="Signin.SSOIdentityListRequest.WaitUntilCacheIsPopulated.Duration"
+    units="ms" expires_after="2021-04-19">
+  <owner>jlebel@chromium.org</owner>
+  <owner>fernandex@chromium.org</owner>
+  <owner>chrome-signin-team@google.com</owner>
+  <summary>
+    Tracks the time between
+    ChromeIdentityServiceImpl::WaitUntilCacheIsPopulated() and the callback is
+    invoked. See: https://crbug.com/897470.
+  </summary>
+</histogram>
+
 <histogram name="Signin.SSOWKWebView.GetAllCookies.CookieCount" units="cookies"
     expires_after="2021-04-19">
   <owner>jlebel@chromium.org</owner>