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 032008b68c3031f8a39bd7f5a6a53beb16e8152f..c2fb3427f8896c3097114f976d63a7e1c6363625 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -47246,6 +47246,24 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Printing.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="Printing.OutOfProcessSubframe" enum="Boolean">

[Alexei Svitkine (slow), 2016/10/12 20:59:47]

Nit: Suggest adding a specific enum for this too.

[Ilya Sherman, 2016/10/12 21:45:54]

As I mentioned on chat, it's probably more appropriate to place these histograms
under an existing top-level category, like "Blink.Printing.*" or
"Renderer.Printing.*" ... or if there's a top-level group name that's typically
used for the OOPIF work, that would be even better.

[nasko, 2016/10/12 21:58:04]

I moved it to PrintPreview, as I think each invocation of window.print() will
result in a print preview window to appear. Creating a new category might not be
needed as I'm not sure if we will need this metric long term.

+  <owner>nasko@chromium.org</owner>
+  <summary>
+    Logged when a document calls the window.print() API. The boolean value
+    indicates whether it is invoked by a subframe in a process different than
+    the top level frame.

[Ilya Sherman, 2016/10/12 21:45:54]

Is this metric recorded only for subframes, or for all frames?  Please clarify
in the description.

[nasko, 2016/10/12 21:58:04]

Tried to reword a bit. Let me know if it is better or how I can clarify it
further.

+  </summary>
+</histogram>
+
 <histogram name="PrintPreview.DestinationAction"
     enum="PrintPreviewPrintDestinationBuckets">
   <owner>vitalybuka@chromium.org</owner>
@@ -73378,6 +73396,11 @@ 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"/>