cc: Allow the main thread to cancel commits

cc/layers/layer_impl.cc

Index: cc/layers/layer_impl.cc
diff --git a/cc/layers/layer_impl.cc b/cc/layers/layer_impl.cc
index eca772371d4559e2092ca7d9f4928a82f24dab2a..3af186b798fbfa4930a19bad6d18de57b6d89a87 100644
--- a/cc/layers/layer_impl.cc
+++ b/cc/layers/layer_impl.cc
@@ -272,6 +272,22 @@ gfx::Vector2dF LayerImpl::ScrollBy(gfx::Vector2dF scroll) {
   return unscrolled;
 }
 
+void LayerImpl::ApplySentScrollDeltas() {
+  // Pending tree never has sent scroll deltas
+  DCHECK(layer_tree_impl()->IsActiveTree());
+
+  // Apply sent scroll deltas to scroll position / scroll delta as if the
+  // main thread had applied them and then committed those values.
+  scroll_offset_ += sent_scroll_delta_;
+  scroll_delta_ -= sent_scroll_delta_;
+  sent_scroll_delta_ = gfx::Vector2d();
+
+  DCHECK_GE(TotalScrollOffset().x(), 0);
+  DCHECK_GE(TotalScrollOffset().y(), 0);
+  DCHECK_LE(TotalScrollOffset().x(), max_scroll_offset_.x());
+  DCHECK_LE(TotalScrollOffset().y(), max_scroll_offset_.y());
+}
+
 InputHandler::ScrollStatus LayerImpl::TryScroll(
     gfx::PointF screen_space_point,
     InputHandler::ScrollInputType type) const {