Add UMA for documents calling window.print().

content/renderer/render_view_impl.cc

Index: content/renderer/render_view_impl.cc
diff --git a/content/renderer/render_view_impl.cc b/content/renderer/render_view_impl.cc
index 6a461f64fb66edfba8a0d3dc9e8e01d9371c4634..7af8c72a39f2866e52d503ee80fee5235ae26fae 100644
--- a/content/renderer/render_view_impl.cc
+++ b/content/renderer/render_view_impl.cc
@@ -1631,6 +1631,15 @@ WebStorageNamespace* RenderViewImpl::createSessionStorageNamespace() {
 }
 
 void RenderViewImpl::printPage(WebLocalFrame* frame) {
+  UMA_HISTOGRAM_BOOLEAN("Printing.InitiatedByScript", frame->top() == frame);
+
+  // Logging whether the top frame is remote is sufficient in this case. If
+  // the top frame is local, the printing code will function correctly and
+  // the frame itself will be printed, so the cases this histogram tracks is
+  // where printing of a subframe will fail as of now.
+  UMA_HISTOGRAM_BOOLEAN("Printing.OutOfProcessSubframe",
+                        frame->top()->isWebRemoteFrame());

[Lei Zhang, 2016/10/11 23:42:59]

frame->top() won't ever be a nullptr, right?

[nasko, 2016/10/11 23:44:17]

Yes, it should never be null, but can be either local or remote. We care about
the cases that the top level frame is a remote one.

[Lei Zhang, 2016/10/11 23:51:14]

Acknowledged.

+
   FOR_EACH_OBSERVER(RenderViewObserver, observers_,
                     PrintPage(frame, input_handler().handling_input_event()));
 }