Add Bookmarks.BookmarksInFolder histogram to bookmark manager.

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 9fe7813d60ec0b07daacd8d46036da29ba10f3a1..3381e70a577fc5d1b369522055ab3d41c887b72a 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -4844,6 +4844,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Bookmarks.BookmarksInFolder">

[Alexei Svitkine (slow), 2016/11/16 00:45:31]

Nit: units="bookmarks"?

[calamity, 2016/11/16 00:52:27]

Done.

+  <owner>yfriedman@chromium.org</owner>

[Alexei Svitkine (slow), 2016/11/16 00:45:31]

You probably want yourself here?

[calamity, 2016/11/16 00:52:27]

Done.

+  <summary>
+    Logs the depth of the bookmark in the bookmark tree hierarchy every time a

[Alexei Svitkine (slow), 2016/11/16 00:45:31]

This description doesn't seem to match the metric name?

[calamity, 2016/11/16 00:52:27]

Done.

+    bookmark is launched. Depth indicates how many levels below a permanent
+    bookmark folder the bookmark was found in (e.g. a bookmark immediately in
+    the bookmark bar has depth 1).
+  </summary>
+</histogram>
+
 <histogram name="Bookmarks.EntryPoint" enum="BookmarksEntryPoint">
   <owner>ianwen@chromium.org</owner>
   <summary>How users add a new bookmark.</summary>