Ignore should-scroll-on-main-thread if main frame is not scrollable

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 72a72b4552545cac9efb7fe390d349c8ddda55d7..206bbc376a7ecced5299838d60fe16ef86857d99 100644
--- a/cc/trees/layer_tree_host_impl_unittest.cc
+++ b/cc/trees/layer_tree_host_impl_unittest.cc
@@ -602,6 +602,27 @@ TEST_F(LayerTreeHostImplTest, NoFlingWhenScrollingOnMain) {
             host_impl_->FlingScrollBegin());
 }
 
+TEST_F(LayerTreeHostImplTest, IgnoreScrollOnMainThreadIfRootNotScrollable) {
+  SetupScrollAndContentsLayers(gfx::Size(100, 100));
+  host_impl_->SetViewportSize(gfx::Size(50, 50));
+  InitializeRendererAndDrawFrame();
+  LayerImpl* root = host_impl_->active_tree()->root_layer();
+
+  root->SetShouldScrollOnMainThread(true);
+  root->SetScrollable(true);
+  root->SetMaxScrollOffset(gfx::Vector2d(10, 10));
+
+  // Makes no different if this is a wheel input event or a gesture.
+  EXPECT_EQ(InputHandler::ScrollOnMainThread,
+            host_impl_->ScrollBegin(gfx::Point(), InputHandler::Wheel));
+
+  // The root is no longer scrollable, so 'should scroll on main thread should
+  // no longer apply'. NB -- we are scrolling a descendant of the root.
+  root->SetMaxScrollOffset(gfx::Vector2d());
+  EXPECT_EQ(InputHandler::ScrollStarted,
+            host_impl_->ScrollBegin(gfx::Point(), InputHandler::Wheel));
+}
+
 TEST_F(LayerTreeHostImplTest, ShouldScrollOnMainThread) {
   SetupScrollAndContentsLayers(gfx::Size(100, 100));
   host_impl_->SetViewportSize(gfx::Size(50, 50));
@@ -615,7 +636,6 @@ TEST_F(LayerTreeHostImplTest, ShouldScrollOnMainThread) {
   EXPECT_EQ(InputHandler::ScrollOnMainThread,
             host_impl_->ScrollBegin(gfx::Point(), InputHandler::Gesture));
 }
-
 TEST_F(LayerTreeHostImplTest, NonFastScrollableRegionBasic) {
   SetupScrollAndContentsLayers(gfx::Size(200, 200));
   host_impl_->SetViewportSize(gfx::Size(100, 100));