Revert of Remove enable/disable wheel gestures setting.

cc/trees/layer_tree_host_impl_unittest.cc

Index: cc/trees/layer_tree_host_impl_unittest.cc
diff --git a/cc/trees/layer_tree_host_impl_unittest.cc b/cc/trees/layer_tree_host_impl_unittest.cc
index 4972bb1b90a42cebaaa3a88fdb04f4c423ce648c..16656d49a71678c2c5856d2cd9ba7e337bc1be9d 100644
--- a/cc/trees/layer_tree_host_impl_unittest.cc
+++ b/cc/trees/layer_tree_host_impl_unittest.cc
@@ -765,16 +765,39 @@
   host_impl_->SetViewportSize(gfx::Size(50, 50));
   DrawFrame();
 
-  // Wheel handlers determine whether mouse events block scroll.
+  // With registered event handlers, wheel scrolls don't necessarily
+  // have to go to the main thread.
   host_impl_->active_tree()->set_event_listener_properties(
       EventListenerClass::kMouseWheel, EventListenerProperties::kBlocking);
-  EXPECT_EQ(
-      EventListenerProperties::kBlocking,
-      host_impl_->GetEventListenerProperties(EventListenerClass::kMouseWheel));
-
-  // But they don't influence the actual handling of the scroll gestures.
   InputHandler::ScrollStatus status =
     host_impl_->ScrollBegin(BeginState(gfx::Point()).get(),
+                                   InputHandler::WHEEL);
+  EXPECT_EQ(InputHandler::SCROLL_ON_MAIN_THREAD, status.thread);
+  EXPECT_EQ(MainThreadScrollingReason::kEventHandlers,
+            status.main_thread_scrolling_reasons);
+
+  host_impl_->active_tree()->set_event_listener_properties(
+      EventListenerClass::kMouseWheel,
+      EventListenerProperties::kBlockingAndPassive);
+  status = host_impl_->ScrollBegin(BeginState(gfx::Point()).get(),
+                                   InputHandler::WHEEL);
+  EXPECT_EQ(InputHandler::SCROLL_ON_MAIN_THREAD, status.thread);
+  EXPECT_EQ(MainThreadScrollingReason::kEventHandlers,
+            status.main_thread_scrolling_reasons);
+
+  // But gesture scrolls can still be handled.
+  status = host_impl_->ScrollBegin(BeginState(gfx::Point()).get(),
+                                   InputHandler::TOUCHSCREEN);
+  EXPECT_EQ(InputHandler::SCROLL_ON_IMPL_THREAD, status.thread);
+  EXPECT_EQ(MainThreadScrollingReason::kNotScrollingOnMain,
+            status.main_thread_scrolling_reasons);
+  host_impl_->ScrollEnd(EndState().get());
+
+  // And if the handlers go away, wheel scrolls can again be processed
+  // on impl.
+  host_impl_->active_tree()->set_event_listener_properties(
+      EventListenerClass::kMouseWheel, EventListenerProperties::kNone);
+  status = host_impl_->ScrollBegin(BeginState(gfx::Point()).get(),
                                    InputHandler::WHEEL);
   EXPECT_EQ(InputHandler::SCROLL_ON_IMPL_THREAD, status.thread);
   EXPECT_EQ(MainThreadScrollingReason::kNotScrollingOnMain,