Record impressions of SignedInAccountsView.

This CL add histogram Signin.SignedInAccountsViewImpression that
counts the number of times the signed-in account view is shown when
the application enters foreground (iOS only).

Bug: 1018211
Change-Id: Iff7f8799a8ecb0b13858d3cb3b744738c01adae7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2106170
Commit-Queue: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753202}

ios/chrome/browser/ui/main/scene_controller.mm

@@ -1485,6 +1485,7 @@ const NSTimeInterval kDisplayPromoDelay = 0.1;
 
 - (void)presentSignedInAccountsViewControllerForBrowserState:
     (ChromeBrowserState*)browserState {
+  UMA_HISTOGRAM_BOOLEAN("Signin.SignedInAccountsViewImpression", true);
   UIViewController* accountsViewController =
       [[SignedInAccountsViewController alloc]
           initWithBrowserState:browserState

tools/metrics/histograms/histograms.xml

@@ -150853,6 +150853,24 @@ OAuth Multilogin call. -->
   </summary>
 </histogram>
 
+<histogram name="Signin.SignedInAccountsViewImpression" units="BooleanShown"
+    expires_after="2020-06-01">
+  <owner>fernandex@chromium.org</owner>
+  <owner>jlebel@chromium.org</owner>
+  <owner>msarda@chromium.org</owner>
+  <summary>
+    Track the number of times the view that displays the accounts that are
+    signed is shown. This view only exists on iOS and is shown every time the
+    application enters forground iff the accounts changed while it was in
+    background. Only bucket &quot;Shown&quot; is being recorded (this histogram
+    is a counter).
+
+    This histogram was added following crbug.com/1006717 (stable regression)
+    where the signed-in accounts view was repeatedly shown for a set of users.
+    It is used as a counter to monitor that there are no bumps in its timeline.
+  </summary>
+</histogram>
+
 <histogram name="Signin.SignedInDurationBeforeSignout" units="minutes"
     expires_after="M77">
   <obsolete>