[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 6e39e7f210b0c3caae848a2da5c053faf580612e..c4dd433dd9529dbe39c753d6b7ff47fae23942ad 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -37683,6 +37683,35 @@ 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
+    a new downloaded page.  The page that is being deleted is counted in this
+    metric because the minimum value of a histogram bucket is &quot;1&quot;.
+  </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">
+  <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>