Fix double subtraction of sent_delta on sync tree activation.

When PushPropertiesTo sets the active layer's delta to delta - sent_delta,
LayerImpl attempts to mirror this to the pending twin. However, in doing
so it again subtracts pending_delta. This patch saves the sent_delta and
clears it before setting the scroll offset and delta on the active layer.

BUG=428327

Review URL: https://codereview.chromium.org/698003006

Cr-Commit-Position: refs/heads/master@{#302709}

cc/layers/layer_impl.cc

@@ -550,10 +550,14 @@ void LayerImpl::PushPropertiesTo(LayerImpl* layer) {
                                                : Layer::INVALID_ID);
   layer->set_user_scrollable_horizontal(user_scrollable_horizontal_);
   layer->set_user_scrollable_vertical(user_scrollable_vertical_);
-  layer->SetScrollOffsetAndDelta(
-      scroll_offset_,
-      layer->ScrollDelta() - layer->sent_scroll_delta());
+
+  // Save the difference but clear the sent delta so that we don't subtract
+  // it again in SetScrollOffsetAndDelta's pending twin mirroring logic.
+  gfx::Vector2dF remaining_delta =
+      layer->ScrollDelta() - layer->sent_scroll_delta();
   layer->SetSentScrollDelta(gfx::Vector2dF());
+  layer->SetScrollOffsetAndDelta(scroll_offset_, remaining_delta);
+
   layer->Set3dSortingContextId(sorting_context_id_);
   layer->SetNumDescendantsThatDrawContent(num_descendants_that_draw_content_);
 

cc/layers/layer_impl_unittest.cc

@@ -434,6 +434,8 @@ class LayerImplScrollTest : public testing::Test {
     return host_impl_.active_tree()->root_layer()->children()[0];
   }
 
+  LayerTreeHostImpl& host_impl() { return host_impl_; }
+
   LayerTreeImpl* tree() { return host_impl_.active_tree(); }
 
   LayerTreeSettings settings() {
@@ -675,6 +677,31 @@ TEST_F(LayerImplScrollTest, ScrollUserUnscrollableLayer) {
   EXPECT_VECTOR_EQ(gfx::Vector2dF(30.5f, 5), layer()->TotalScrollOffset());
 }
 
+TEST_F(LayerImplScrollTest, PushPropertiesToMirrorsTotalScrollOffset) {
+  gfx::ScrollOffset scroll_offset(10, 5);
+  gfx::Vector2dF scroll_delta(12, 18);
+
+  host_impl().CreatePendingTree();
+
+  layer()->SetScrollOffset(scroll_offset);
+  gfx::Vector2dF unscrolled = layer()->ScrollBy(scroll_delta);
+
+  EXPECT_VECTOR_EQ(gfx::Vector2dF(0, 0), unscrolled);
+  EXPECT_VECTOR_EQ(gfx::Vector2dF(22, 23), layer()->TotalScrollOffset());
+
+  layer()->SetSentScrollDelta(scroll_delta);
+
+  scoped_ptr<LayerImpl> pending_layer =
+      LayerImpl::Create(host_impl().sync_tree(), layer()->id());
+  pending_layer->SetScrollOffset(layer()->TotalScrollOffset());
+
+  pending_layer->PushPropertiesTo(layer());
+
+  EXPECT_VECTOR_EQ(gfx::Vector2dF(22, 23), layer()->TotalScrollOffset());
+  EXPECT_VECTOR_EQ(layer()->TotalScrollOffset(),
+                   pending_layer->TotalScrollOffset());
+}
+
 TEST_F(LayerImplScrollTest, SetNewScrollbarParameters) {
   gfx::ScrollOffset scroll_offset(10, 5);
   layer()->SetScrollOffset(scroll_offset);