Make mouse wheel events trigger WebContentsObserver::DidGetUserInteraction.

content/public/browser/web_contents_observer.h

Index: content/public/browser/web_contents_observer.h
diff --git a/content/public/browser/web_contents_observer.h b/content/public/browser/web_contents_observer.h
index 37f28b1191abe06936caf47801ade9828344e210..860e5b6a4f633336d253d6db698f17e74fe027b2 100644
--- a/content/public/browser/web_contents_observer.h
+++ b/content/public/browser/web_contents_observer.h
@@ -319,8 +319,12 @@ class CONTENT_EXPORT WebContentsObserver : public IPC::Listener,
   virtual void DidGetUserGesture() {}
 
   // Called when there has been direct user interaction with the WebContents.
-  // Direct user input includes 1) any mouse down event; 2) any raw key down
-  // event; and 3) any gesture tap event (including taps and scrolls).
+  // The type argument specifies the kind of interaction. Direct user input
+  // signalled through this callback includes:
+  // 1) any mouse down event (blink::WebInputEvent::MouseDown);
+  // 2) the start of a mouse wheel scroll (blink::WebInputEvent::MouseWheel);

[nasko, 2015/11/03 20:45:59]

It isn't really the start, is it? It is the first even in an interval. If I
scroll really slowly, such that each event is delivered right after the time
elapsed, wouldn't it result in another event fired?

[dominickn, 2015/11/03 21:29:42]

Yes. I've updated this comment to match.

+  // 3) any raw key down event (blink::WebInputEvent::RawKeyDown); and
+  // 4) any gesture tap event (blink::WebInputEvent::GestureTapDown).
   virtual void DidGetUserInteraction(const blink::WebInputEvent::Type type) {}
 
   // This method is invoked when a RenderViewHost of this WebContents was