Fix IPH (in product help) for Feed V2

IPH triggering was relying on the onScrollStateChanged() method,
but feed V2 was not passing along onScrollStateChanged.  This change
routes the message to listeners.

Bug: 1113468
Change-Id: I087915022087928c33135e5a71e712f3964a1d2a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2340366Reviewed-by: Vincent Boisselle <vincb@google.com>
Reviewed-by: Dan H <harringtond@chromium.org>
Commit-Queue: Peter Williamson <petewil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795494}

chrome/android/feed/core/java/src/org/chromium/chrome/browser/feed/v2/FeedStream.java

@@ -219,6 +219,12 @@ public class FeedStream implements Stream {
                     listener.onScrolled(dx, dy);
                 }
             }
+            @Override
+            public void onScrollStateChanged(RecyclerView v, int newState) {
+                for (ScrollListener listener : mScrollListeners) {
+                    listener.onScrollStateChanged(newState);
+                }
+            }
         });
     }