Add UseCounter for weird body+viewport scrolling

third_party/WebKit/Source/core/dom/Document.cpp

Index: third_party/WebKit/Source/core/dom/Document.cpp
diff --git a/third_party/WebKit/Source/core/dom/Document.cpp b/third_party/WebKit/Source/core/dom/Document.cpp
index 681c3f1294106099e26e9b5e690e39038250a3bb..8635fbebf2b65f4c2d861b942056491fd9ece6fb 100644
--- a/third_party/WebKit/Source/core/dom/Document.cpp
+++ b/third_party/WebKit/Source/core/dom/Document.cpp
@@ -1637,6 +1637,12 @@ void Document::inheritHtmlAndBodyElementStyles(StyleRecalcChange change)
         } else {
             ASSERT(element == documentElement());
             overflowStyle = documentElementStyle.get();
+
+            // The body element has its own scrolling box, independent from the viewport.
+            // This is a bit of a weird edge case in the CSS spec that we might want to try to
+            // eliminate some day (eg. for ScrollTopLeftInterop - see http://crbug.com/157855).
+            if (bodyStyle && !bodyStyle->isOverflowVisible())
+                UseCounter::count(*this, UseCounter::BodyScrollsInAdditionToViewport);
         }
     }