Add IOS.RestoreNavigationItemTime to histograms.xml.

UMA logging added here:
https://chromium-review.googlesource.com/1232615

Bug: 877671
Change-Id: Icf550ec0dd047462fcf385a7fbf830099c487407
Reviewed-on: https://chromium-review.googlesource.com/1238470
Commit-Queue: Eugene But <eugenebut@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595503}

tools/metrics/histograms/histograms.xml

@@ -39280,6 +39280,18 @@ uploading your change for review.
   </summary>
 </histogram>
 
+<histogram name="IOS.RestoreNavigationItemTime" units="ms">
+  <owner>eugenebut@chromium.org</owner>
+  <summary>
+    [iOS] Time spent on restoring committed Navigation Manager's items. Recorded
+    once per restoration, as a cumulative time across all items. Restoration is
+    triggered in the following cases (the list is not exhaustive): app cold
+    start, remote tab was open, cookies were cleared, recently closed tab was
+    restored. This metric will be used for monitoring session restoration
+    performance which relies on system's WKWebView.
+  </summary>
+</histogram>
+
 <histogram name="IOS.SearchExtension.Action" enum="IOSSearchExtensionAction">
   <owner>olivierrobin@chromium.org</owner>
   <summary>The action selected by the user in the Search Extension.</summary>