Add font substitution metric for Linux

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 eddd628c65a3930b7f1b68dbb892bade47c64339..a958ecc98cc6565c2f3760cddff0828695190ce2 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -41632,6 +41632,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="PDF.FontSubstituted" enum="PDFFontSubstituted">

[rkaplow, 2016/10/07 17:40:31]

I would just use Boolean, and name this something like 
PDF.IsFontSubstituted, so you don't need custom enum here

+  <owner>npm@chromium.org</owner>
+  <summary>
+    Tracks documents opened in the PDF viewer where substitute fonts need to be
+    used.
+  </summary>
+</histogram>
+
 <histogram name="Pepper.InterfaceUsed" enum="PepperInterface">
   <owner>sehr@chromium.org</owner>
   <owner>bradnelson@chromium.org</owner>
@@ -92899,6 +92907,11 @@ value.
   <int value="2" label="Has Bookmarks"/>
 </enum>
 
+<enum name="PDFFontSubstituted" type="int">
+  <int value="0" label="No Font Subst"/>
+  <int value="1" label="Font Substituted"/>
+</enum>
+
 <enum name="PeerConnectionCounters" type="int">
   <int value="0" label="PeerConnection enabled with IPv4."/>
   <int value="1" label="PeerConnection enabled with Ipv6."/>