Refactor delegated scrolling in LayerImpl

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 979f1a9dd30ee329fda804e14fbd409463e278f4..e4bb7dc55cd3bf093d754771713017b269c0f61d 100644
--- a/cc/trees/layer_tree_host_impl_unittest.cc
+++ b/cc/trees/layer_tree_host_impl_unittest.cc
@@ -3972,6 +3972,7 @@ TEST_F(LayerTreeHostImplTest, RootLayerScrollOffsetDelegation) {
   scroll_delegate.set_getter_return_value(current_offset);
   EXPECT_EQ(InputHandler::SCROLL_STARTED,
             host_impl_->ScrollBegin(gfx::Point(), InputHandler::GESTURE));
+  host_impl_->OnRootLayerDelegatedScrollOffsetChanged();
 
   host_impl_->ScrollBy(gfx::Point(), scroll_delta);
   EXPECT_EQ(ScrollOffsetWithDelta(current_offset, scroll_delta),
@@ -3979,11 +3980,13 @@ TEST_F(LayerTreeHostImplTest, RootLayerScrollOffsetDelegation) {
 
   current_offset = gfx::ScrollOffset(42.f, 41.f);
   scroll_delegate.set_getter_return_value(current_offset);
+  host_impl_->OnRootLayerDelegatedScrollOffsetChanged();
   host_impl_->ScrollBy(gfx::Point(), scroll_delta);
   EXPECT_EQ(current_offset + gfx::ScrollOffset(scroll_delta),
             scroll_delegate.last_set_scroll_offset());
   host_impl_->ScrollEnd();
   scroll_delegate.set_getter_return_value(gfx::ScrollOffset());
+  host_impl_->OnRootLayerDelegatedScrollOffsetChanged();
 
   // Forces a full tree synchronization and ensures that the scroll delegate
   // sees the correct size of the new tree.
@@ -3997,6 +4000,7 @@ TEST_F(LayerTreeHostImplTest, RootLayerScrollOffsetDelegation) {
   // the root scrollable layer.
   current_offset = gfx::ScrollOffset(13.f, 12.f);
   scroll_delegate.set_getter_return_value(current_offset);
+  host_impl_->OnRootLayerDelegatedScrollOffsetChanged();
   host_impl_->SetRootLayerScrollOffsetDelegate(NULL);
 
   EXPECT_EQ(current_offset.ToString(),