[Offline Pages] Adds duplicate detection metrics.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 3fc83649c354de13a464973b462a011da84df924..1be21346b0237744caab44fa0cd571bdd53c9c7d 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -37734,6 +37734,35 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <summary>Result of removing an offline copy for a page.</summary>
 </histogram>
 
+<histogram name="OfflinePages.DownloadDeletedPageDuplicateCount" units="pages">
+  <owner>dewittj@chromium.org</owner>
+  <owner>dimich@chromium.org</owner>
+  <summary>
+    How many downloaded pages with the same URL exist at the time that we delete

[Mark P, 2016/09/09 18:41:05]

nit: How many -> The number of
(sounds less like a question)

ditto below

[Mark P, 2016/09/09 18:41:05]

nit: exist -> that exist
ditto below

[dewittj, 2016/09/12 18:22:50]

Done.

[dewittj, 2016/09/12 18:22:50]

Done.

+    a new downloaded page.  The page that is being deleted is counted in this

[Mark P, 2016/09/09 18:41:05]

nit: omit "new" -- I don't think it adds anything here.  Furthermore, I don't
think you're restricting this metric to only be emitted when a user deletes
downloaded  pages that are downloaded recently (new).
ditto below

[dewittj, 2016/09/12 18:22:50]

Done.

+    metric because the minimum value of a histogram bucket is "1".

[Mark P, 2016/09/09 18:41:05]

The "because" part of this statement is wrong.  Every histogram includes an
"underflow" bucket to capture emits of 0.  If you want to include the page being
deleted in this metric, you can, but it's not necessary.  For what it's worth, I
think this metric would make more sense if you don't.

[dewittj, 2016/09/12 18:22:50]

Done.

+  </summary>
+</histogram>
+
+<histogram name="OfflinePages.DownloadSavedPageDuplicateCount" units="pages">
+  <owner>dewittj@chromium.org</owner>
+  <owner>dimich@chromium.org</owner>
+  <summary>
+    How many downloaded pages with the same URL exist at the time that we save a
+    new downloaded page.
+  </summary>
+</histogram>
+
+<histogram name="OfflinePages.DownloadSavedPageTimeSinceDuplicateSaved">

[Mark P, 2016/09/09 18:41:05]

units="ms"?

[dewittj, 2016/09/12 18:22:50]

Done.

+  <owner>dewittj@chromium.org</owner>
+  <owner>dimich@chromium.org</owner>
+  <summary>
+    If at save time there is another downloaded page with the same URL, this
+    tracks this amount of time between creation of the most recent existing page
+    and the current page.
+  </summary>
+</histogram>
+
 <histogram name="OfflinePages.Edit.BookmarkUrlChangedForOfflinePage"
     enum="BooleanMatched">
   <obsolete>