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 4865fe2bd2c4376e93b9248ac1f6e0bdb37a0780..95d1da7a61fb5c6a03a220a7355e591d0ea213e4 100644
--- a/cc/layers/layer_impl.cc
+++ b/cc/layers/layer_impl.cc
@@ -265,11 +265,24 @@ gfx::Vector2dF LayerImpl::ScrollBy(gfx::Vector2dF scroll) {
   new_delta.SetToMax(min_delta);
   new_delta.SetToMin(max_delta);
   gfx::Vector2dF unscrolled = ScrollDelta() + scroll - new_delta;
-
   SetScrollDelta(new_delta);
   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.
+  gfx::Vector2dF previous_scroll_delta = ScrollDelta();
+  gfx::Vector2d previous_sent_scroll_delta = sent_scroll_delta_;
+
+  SetSentScrollDelta(gfx::Vector2d());
+  SetScrollOffset(scroll_offset_ + previous_sent_scroll_delta);
+  SetScrollDelta(previous_scroll_delta - previous_sent_scroll_delta);
+}
+
 InputHandler::ScrollStatus LayerImpl::TryScroll(
     gfx::PointF screen_space_point,
     InputHandler::ScrollInputType type) const {
@@ -893,8 +906,13 @@ void LayerImpl::SetScrollDelta(gfx::Vector2dF scroll_delta) {
   } else {
     scroll_delta_ = scroll_delta;
   }
-  NoteLayerPropertyChangedForSubtree();
 
+  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());
+
+  NoteLayerPropertyChangedForSubtree();
   UpdateScrollbarPositions();
 }