[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..7448893eccee5cd1c9c5c58f7499bbaa29127382 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>
+    The number of downloaded pages with the same URL that exist at the time that
+    we delete a downloaded page.

[Mark P, 2016/09/12 18:32:09]

You corrected the description here to remove the the statement "The page that is
being deleted is counted in this metric." yet I don't see a code change
elsewhere in this patchset.

[dewittj, 2016/09/12 18:36:00]

When I looked back at the code I found that the description was just wrong, must
have been a product of iteration on the metric before review.

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

[Mark P, 2016/09/12 18:32:09]

No, I think it should be seconds.  You're still using histogram times, which
emits in seconds.  You only changed your minimum bucket to be 1 second
(=1000ms).

[dewittj, 2016/09/12 18:36:00]

Isn't it now in seconds?

[Mark P, 2016/09/12 18:52:10]

No, you're using the UMA_HISTOGRAM_CUSTOM_TIMES, which, if you dig into it,
calls AddTime(sample), which emits an entry with the time in milliseconds.
https://cs.chromium.org/chromium/src/base/metrics/histogram_base.cc?dr=C&sq=p...

Your previous revisions simply changed the minimum and maximum bucket values and
bucket count.

+  <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>