Handle nested position:sticky elements

cc/blink/web_layer_impl.cc

Index: cc/blink/web_layer_impl.cc
diff --git a/cc/blink/web_layer_impl.cc b/cc/blink/web_layer_impl.cc
index 73e62b3e66fc0f018aa19bf2484402d08a1b69af..ad18b365ea4bd92e683dcc4dac1bd12430048595 100644
--- a/cc/blink/web_layer_impl.cc
+++ b/cc/blink/web_layer_impl.cc
@@ -378,6 +378,9 @@ ToWebLayerStickyPositionConstraint(
       constraint.scroll_container_relative_sticky_box_rect;
   web_constraint.scrollContainerRelativeContainingBlockRect =
       constraint.scroll_container_relative_containing_block_rect;
+  // TODO(smcgruer): Copy over ancestor pointers too.

[smcgruer, 2017/02/14 17:02:06]

flackr : Ali suggested switching from layer pointers to layer ids, as it seemed
like the only way to copy back from cc to blink. Thoughts? Another option would
be to leave this method unfulfilled, as it is only used for tests afaik.

[flackr, 2017/02/21 00:32:42]

It would be nice to be able to test these constraints back on the blink side and
to not lose anything in the conversion back and forth but if it's causing
nontrivial issues, I would be okay with deferring this until later (i.e. file
and reference a bug with TODO) to keep this patch simple.

+  web_constraint.cachedComputedStickyOffset =
+      blink::WebFloatSize(constraint.cached_computed_sticky_offset);
   return web_constraint;
 }
 static cc::LayerStickyPositionConstraint ToStickyPositionConstraint(
@@ -398,6 +401,16 @@ static cc::LayerStickyPositionConstraint ToStickyPositionConstraint(
       web_constraint.scrollContainerRelativeStickyBoxRect;
   constraint.scroll_container_relative_containing_block_rect =
       web_constraint.scrollContainerRelativeContainingBlockRect;
+  constraint.nearest_layer_shifting_sticky_box =
+      web_constraint.nearestLayerShiftingStickyBox
+          ? web_constraint.nearestLayerShiftingStickyBox->ccLayer()
+          : nullptr;
+  constraint.nearest_layer_shifting_containing_block =
+      web_constraint.nearestLayerShiftingContainingBlock
+          ? web_constraint.nearestLayerShiftingContainingBlock->ccLayer()
+          : nullptr;
+  constraint.cached_computed_sticky_offset =
+      web_constraint.cachedComputedStickyOffset;
   return constraint;
 }
 void WebLayerImpl::setStickyPositionConstraint(