Add PDF.PageCount to histograms.xml.

This metric is already logged but was never added to histograms.xml.

Bug: chromium:768986
Change-Id: I77322aa2e4a71469c45a32bb1cc88aac9f42c7db
Reviewed-on: https://chromium-review.googlesource.com/714125
Commit-Queue: Henrique Nakashima <hnakashima@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509924}

tools/metrics/histograms/histograms.xml

@@ -58210,6 +58210,13 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="PDF.PageCount" units="pages">
+  <owner>hnakashima@chromium.org</owner>
+  <summary>
+    Tracks the number of pages in PDF documents opened in the PDF viewer.
+  </summary>
+</histogram>
+
 <histogram name="Pepper.Graphics3DHasShareGroup" units="BooleanShareGroup">
   <owner>jbauman@chromium.org</owner>
   <summary>