Arrange compositor scrolling into scroll customization format

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 5c20611b03f6695fbdb6c2be402ebbfd864fcdff..957bd2cee503e3dac5a6c459ae1a3d74c5ffddb9 100644
--- a/cc/layers/layer_impl.cc
+++ b/cc/layers/layer_impl.cc
@@ -17,6 +17,7 @@
 #include "cc/debug/layer_tree_debug_state.h"
 #include "cc/debug/micro_benchmark_impl.h"
 #include "cc/debug/traced_value.h"
+#include "cc/input/scroll_state.h"
 #include "cc/layers/layer_utils.h"
 #include "cc/layers/painted_scrollbar_layer_impl.h"
 #include "cc/output/copy_output_request.h"
@@ -197,6 +198,30 @@ void LayerImpl::SetScrollChildren(std::set<LayerImpl*>* children) {
   SetNeedsPushProperties();
 }
 
+void LayerImpl::DistributeScroll(ScrollState* scroll_state) {
+  DCHECK(scroll_state);
+  if (scroll_state->FullyConsumed())
+    return;
+
+  scroll_state->DistributeToScrollChainDescendant();
+
+  // If the scroll doesn't propagate, and we're currently scrolling
+  // a layer other than this one, prevent the scroll from
+  // propagating to this layer.
+  if (!scroll_state->should_propagate() &&
+      scroll_state->delta_consumed_for_scroll_sequence() &&

[Ian Vollick, 2015/07/30 19:02:25]

Hmm, what if we shouldn't propagate, and the true scrolling layer (that isn't
us) decides not to consume any scroll delta. Won't this cause us to ApplyScroll
incorrectly here? Or by not consuming any scrolling is it relinquishing the
right to be the scrolling layer?

[tdresser, 2015/07/30 20:34:20]

delta_consumed_for_scroll_sequence persists for the duration of the scroll.

The only case where we shouldn't propagate is within a touchscreen fling. In
this case, we want to scroll the very first scroller that scrolls, not
necessarily the innermost scroller.

So essentially yes, but not consuming any scroll it's relinquishing the right to
be the scrolling layer.

+      scroll_state->current_native_scrolling_layer() != this) {
+    return;
+  }
+
+  ApplyScroll(scroll_state);
+}
+
+void LayerImpl::ApplyScroll(ScrollState* scroll_state) {
+  DCHECK(scroll_state);
+  layer_tree_impl()->ApplyScroll(this, scroll_state);
+}
+
 void LayerImpl::SetNumDescendantsThatDrawContent(int num_descendants) {
   if (num_descendants_that_draw_content_ == num_descendants)
     return;