Material PDF: Add metrics to count usage of PDF title and bookmarks

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 e066f2787a9bca3c4c26a30dd47318b5e1ae3c14..0a3f6d4076bbcc843ac9987a3e6bba46e1b27e28 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -30825,6 +30825,13 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="PDF.HasFeature" enum="Boolean">
+  <owner>tsergeant@chromium.org</owner>
+  <summary>
+    Tracks which features are used by documents opened in the PDF viewer.

[Alexei Svitkine (slow), 2015/10/14 15:06:10]

Nit: Mention when it's logged.

+  </summary>
+</histogram>
+
 <histogram name="Pepper.InterfaceUsed" enum="PepperInterface">
   <owner>mackinlay@google.com</owner>
   <owner>teravest@chromium.org</owner>
@@ -78398,6 +78405,12 @@ To add a new entry, add it with any value and run test to compute valid value.
   <affected-histogram name="PasswordManager.TotalAccountsHiRes.WithScheme"/>
 </histogram_suffixes>
 
+<histogram_suffixes name="PDFFeatures" separator=".">
+  <suffix name="Title" label="PDF document has a title field."/>
+  <suffix name="Bookmarks" label="PDF document has a table of contents."/>

[Alexei Svitkine (slow), 2015/10/14 15:06:10]

Hmm, while this way of doing it is fine, I wonder if using a single enumerated
histogram with an enum value for each feature would be sufficient as well? If it
would, I think that would be a better approach since it would be simpler and
more efficient.

You can have an enum entry in that histogram that is recorded unconditionally in
the same place, which can be used as a baseline. 

+  <affected-histogram name="PDF.HasFeature"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="PerformanceMonitor" separator=".">
   <suffix name="BrowserProcess"/>
   <suffix name="GPUProcess"/>