Add units to Net.RequestTime2Success histograms

Followup to https://chromium-review.googlesource.com/c/chromium/src/+/800950

Bug: None
Change-Id: If1bbf18575ca1b0562499c899960ac003a7678c9
Reviewed-on: https://chromium-review.googlesource.com/802474
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Commit-Queue: Steven Holte <holte@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520810}

tools/metrics/histograms/histograms.xml

@@ -45733,7 +45733,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
-<histogram name="Net.RequestTime2Success.MainFrame">
+<histogram name="Net.RequestTime2Success.MainFrame" units="ms">
   <owner>csharrison@chromium.org</owner>
   <summary>
     The amount of time between request initiation and request completion for
@@ -45741,7 +45741,7 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
-<histogram name="Net.RequestTime2Success.Subresource">
+<histogram name="Net.RequestTime2Success.Subresource" units="ms">
   <owner>csharrison@chromium.org</owner>
   <summary>
     The amount of time between request initiation and request completion for