Changed units of OfflineIndicator.ShownDuration from seconds to

milliseconds to match the data being recorded.

The stated units of OfflineIndicator.ShownDuration were seconds[1], but
the data being reported was in milliseconds[2]. The Java interface for
recording time histograms requires the durations to be stored in longs
with units of milliseconds, so this change switches the units of the
histogram to match the reported data.

[1] https://uma.googleplex.com/p/chrome/histograms?sid=005d3b5589e4a01a7df40fefea7a5b3c
[2] https://source.chromium.org/chromium/chromium/src/+/master:chrome/android/java/src/org/chromium/chrome/browser/offlinepages/indicator/OfflineIndicatorControllerV2.java;l=142

Bug: 1151504
Change-Id: Ie55abc479d29d048201036c8c0b176f0f9f2f836
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2552896Reviewed-by: Theresa  <twellington@chromium.org>
Reviewed-by: Sinan Sahin <sinansahin@google.com>
Reviewed-by: Weilun Shi <sweilun@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Max Curran <curranmax@chromium.org>
Cr-Commit-Position: refs/heads/master@{#832482}

tools/metrics/histograms/histograms_xml/offline/histograms.xml

@@ -57,7 +57,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
   </summary>
 </histogram>
 
-<histogram name="OfflineIndicator.ShownDuration" units="s"
+<histogram name="OfflineIndicator.ShownDuration" units="ms"
     expires_after="2021-05-09">
   <owner>curranmax@chromium.org</owner>
   <owner>tbansal@chromium.org</owner>