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..afaa581653f3f8655b4ce73ab2861f10e86db456 100644
--- a/content/public/browser/web_contents_observer.h
+++ b/content/public/browser/web_contents_observer.h
@@ -319,8 +319,16 @@ 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);
+  // 3) any raw key down event (blink::WebInputEvent::RawKeyDown); and
+  // 4) any gesture tap event (blink::WebInputEvent::GestureTapDown).
+  // The start of a mouse wheel scroll is heuristically detected: a mouse
+  // wheel event fired at least 0.1 seconds after any other wheel event is
+  // regarded as the beginning of a scroll. This matches the interval used by
+  // the Blink EventHandler to detect the end of scrolls.
   virtual void DidGetUserInteraction(const blink::WebInputEvent::Type type) {}
 
   // This method is invoked when a RenderViewHost of this WebContents was