Made layout viewport scroll updates from compositor work like ordinary layers.

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 77e6f798ad6fcf2c208331c1af02208300cf216e..4d07869bd838fe37ea707340ca1f044814237780 100644
--- a/cc/trees/layer_tree_host_impl_unittest.cc
+++ b/cc/trees/layer_tree_host_impl_unittest.cc
@@ -261,6 +261,15 @@ class LayerTreeHostImplTest : public testing::Test,
       times_encountered++;
     }
 
+    if (id == scroll_info.inner_viewport_scroll.layer_id) {
+      if (scroll_delta != scroll_info.inner_viewport_scroll.scroll_delta) {
+        return ::testing::AssertionFailure()
+               << "Expected " << scroll_delta.ToString() << ", not "
+               << scroll_info.inner_viewport_scroll.scroll_delta.ToString();
+      }
+      times_encountered++;
+    }
+
     if (times_encountered != 1)
       return ::testing::AssertionFailure() << "No layer found with id " << id;
     return ::testing::AssertionSuccess();