cc: Move SyncedScrollOffset to scroll tree

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 32e461186d8abe9673795556b86ac7c6d0ee9ba9..13addc76b8e94381507e4315e55615dd15e158ef 100644
--- a/cc/trees/layer_tree_host_impl_unittest.cc
+++ b/cc/trees/layer_tree_host_impl_unittest.cc
@@ -2033,7 +2033,7 @@ TEST_F(LayerTreeHostImplTest, PinchGesture) {
     host_impl_->active_tree()->PushPageScaleFromMainThread(1.f, min_page_scale,
                                                            max_page_scale);
     scroll_layer->SetScrollDelta(gfx::Vector2d());
-    scroll_layer->PullDeltaForMainThread();
+    scroll_layer->synced_scroll_offset()->PullDeltaForMainThread();
     scroll_layer->PushScrollOffsetFromMainThread(gfx::ScrollOffset(50, 50));
 
     float page_scale_delta = 0.1f;
@@ -2056,7 +2056,7 @@ TEST_F(LayerTreeHostImplTest, PinchGesture) {
     host_impl_->active_tree()->PushPageScaleFromMainThread(1.f, min_page_scale,
                                                            max_page_scale);
     scroll_layer->SetScrollDelta(gfx::Vector2d());
-    scroll_layer->PullDeltaForMainThread();
+    scroll_layer->synced_scroll_offset()->PullDeltaForMainThread();
     scroll_layer->PushScrollOffsetFromMainThread(gfx::ScrollOffset(20, 20));
 
     float page_scale_delta = 1.f;
@@ -2079,7 +2079,7 @@ TEST_F(LayerTreeHostImplTest, PinchGesture) {
     host_impl_->active_tree()->PushPageScaleFromMainThread(1.f, min_page_scale,
                                                            max_page_scale);
     scroll_layer->SetScrollDelta(gfx::Vector2d());
-    scroll_layer->PullDeltaForMainThread();
+    scroll_layer->synced_scroll_offset()->PullDeltaForMainThread();
     scroll_layer->PushScrollOffsetFromMainThread(gfx::ScrollOffset(20, 20));
 
     float page_scale_delta = 1.f;
@@ -2104,7 +2104,7 @@ TEST_F(LayerTreeHostImplTest, PinchGesture) {
   {
     host_impl_->active_tree()->PushPageScaleFromMainThread(0.5f, 0.5f, 4.f);
     scroll_layer->SetScrollDelta(gfx::Vector2d());
-    scroll_layer->PullDeltaForMainThread();
+    scroll_layer->synced_scroll_offset()->PullDeltaForMainThread();
     scroll_layer->PushScrollOffsetFromMainThread(gfx::ScrollOffset(0, 0));
 
     host_impl_->ScrollBegin(BeginState(gfx::Point(0, 0)).get(),
@@ -10192,12 +10192,17 @@ TEST_F(LayerTreeHostImplTest, JitterTest) {
 
     LayerTreeImpl* pending_tree = host_impl_->pending_tree();
     pending_tree->PushPageScaleFromMainThread(1.f, 1.f, 1.f);
+    LayerImpl* last_scrolled_layer = pending_tree->LayerById(
+        host_impl_->active_tree()->LastScrolledLayerId());
+    gfx::ScrollOffset pending_base =
+        last_scrolled_layer->synced_scroll_offset()->PendingBase();
     pending_tree->property_trees()->needs_rebuild = true;
     pending_tree->BuildPropertyTreesForTesting();
+    last_scrolled_layer->synced_scroll_offset()->PushFromMainThread(
+        pending_base);

[ajuma, 2016/03/03 18:05:06]

The way this logic worked before relied on the pending and active layers sharing
the same persistent SyncedProperty instance. But now we get a new SyncedProperty
instance each time we build property trees. Do you also need to make sure that
the active_base_ and active_delta_ (not just the pending_base_) get restored to
the previous value? Also, please add a comment explaining why the scroll offset
needs to be updated this way.

[sunxd, 2016/03/03 18:50:13]

I think we don't need to. ScrollTree only clear itself when built from main
thread, when building from impl thread, it calls synced_scroll_offset to do the
updates where synced_scroll_offset can fetch the existing offset.

+
     pending_tree->set_needs_update_draw_properties();
     pending_tree->UpdateDrawProperties(false);
-    LayerImpl* last_scrolled_layer = pending_tree->LayerById(
-        host_impl_->active_tree()->LastScrolledLayerId());
     float jitter =
         LayerTreeHostCommon::CalculateFrameJitter(last_scrolled_layer);
     // There should not be any jitter measured till we hit the fixed point hits