PDF: Add UMA to track successful / failed PDF loads

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 0651c7279e03e617a7261b16745db3d3d85cbe1a..fe8bd5ad8fbbc7fd550a83f8659290936d7e4780 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -49471,6 +49471,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="PDF.LoadStatus" enum="PDFLoadStatus">
+  <owner>tommycli@chromium.org</owner>
+  <summary>
+    Tracks what happens when Chrome tries to load a PDF. This metric tracks all
+    cases, but is focused on measuring failed PDF embeds, which occur if the
+    user is on Android, or if PDF plugin is disabled.
+  </summary>
+</histogram>
+
 <histogram name="Pepper.Graphics3DHasShareGroup" units="BooleanShareGroup">
   <owner>jbauman@chromium.org</owner>
   <summary>
@@ -109272,6 +109281,12 @@ from previous Chrome versions.
   <int value="2" label="Has Bookmarks"/>
 </enum>
 
+<enum name="PDFLoadStatus" type="int">
+  <int value="0" label="Loaded a full-page PDF with PDFium"/>
+  <int value="1" label="Loaded an embedded PDF with PDFium"/>
+  <int value="2" label="Showed disabled plugin placeholder for embedded PDF"/>
+</enum>
+
 <enum name="PeerConnectionCounters" type="int">
   <int value="0" label="PeerConnection enabled with IPv4."/>
   <int value="1" label="PeerConnection enabled with Ipv6."/>