Update units on SafeBrowsing.DelayedWarnings.TimeOnPage

This changes the units from "seconds" to "ms", since the underlying
histogram time functions log TimeDelta data in milliseconds. This
doesn't affect the logged data, only how it displays on the dashboard,
so we don' t need to create a new histogram for this and can just
update the unit.

Change-Id: I8df03c321e2c9f92aa0f47750715ab3e22ef234c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2522331
Auto-Submit: Chris Thompson <cthomp@chromium.org>
Reviewed-by: Mustafa Emre Acer <meacer@chromium.org>
Commit-Queue: Chris Thompson <cthomp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824662}

tools/metrics/histograms/histograms_xml/safe_browsing/histograms.xml

@@ -614,7 +614,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
   </summary>
 </histogram>
 
-<histogram name="SafeBrowsing.DelayedWarnings.TimeOnPage" units="seconds"
+<histogram name="SafeBrowsing.DelayedWarnings.TimeOnPage" units="ms"
     expires_after="M89">
   <owner>meacer@chromium.org</owner>
   <owner>estark@chromium.org</owner>
@@ -630,7 +630,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="SafeBrowsing.DelayedWarnings.TimeOnPage_UrlElisionDisabled"
-    units="seconds" expires_after="M89">
+    units="ms" expires_after="M89">
   <owner>meacer@chromium.org</owner>
   <owner>estark@chromium.org</owner>
   <owner>chrome-safebrowsing-alerts@google.com</owner>