Add UMA for documents calling window.print().

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 cf1ba12587959bc77a5d652ac2be0247049f0f60..2227f6c9a745a2408fab3edebe7fc7ca777d2c52 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -47337,6 +47337,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="PrintPreview.InitiatedByScript" enum="BooleanMainFrame">
+  <owner>nasko@chromium.org</owner>
+  <summary>
+    Logged when a document calls the window.print() API. The boolean value
+    indicates whether it is invoked by the main frame. It will be false for
+    documents in subframes.
+  </summary>
+</histogram>
+
 <histogram name="PrintPreview.ManagePrinters">
   <owner>vitalybuka@chromium.org</owner>
   <summary>
@@ -47351,6 +47360,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="PrintPreview.OutOfProcessSubframe" enum="Boolean">
+  <owner>nasko@chromium.org</owner>
+  <summary>
+    Logged when a document calls the window.print() API in any frame. The
+    boolean value will be true when the API call is invoked by a document which
+    is in a process different than the top level document. It will be false in
+    all other cases.
+  </summary>
+</histogram>
+
 <histogram name="PrintPreview.PageCount.Initial">
   <owner>vitalybuka@chromium.org</owner>
   <summary>
@@ -73428,6 +73447,11 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="1" label="Loaded"/>
 </enum>
 
+<enum name="BooleanMainFrame" type="int">
+  <int value="0" label="Subframe"/>
+  <int value="1" label="Main frame"/>
+</enum>
+
 <enum name="BooleanMatched" type="int">
   <int value="0" label="Not matched"/>
   <int value="1" label="Matched"/>