Add SurfaceWillDraw notification

cc/surfaces/surface_aggregator.cc

Index: cc/surfaces/surface_aggregator.cc
diff --git a/cc/surfaces/surface_aggregator.cc b/cc/surfaces/surface_aggregator.cc
index e19a997541719faaeefea65b62c24b2873e2e78f..a17440557d38cdee838ec0c3d3218fc3454b6a03 100644
--- a/cc/surfaces/surface_aggregator.cc
+++ b/cc/surfaces/surface_aggregator.cc
@@ -762,8 +762,10 @@ gfx::Rect SurfaceAggregator::PrewalkTree(const SurfaceId& surface_id,
   CHECK(debug_weak_this.get());
   // TODO(staraz): It shouldn't need to call the callback when the damage is
   // from |surface| and not from |child_surfaces|.
-  if (!damage_rect.IsEmpty())
+  if (!damage_rect.IsEmpty()) {
     surface->RunWillDrawCallback(damage_rect);
+    manager_->SurfaceWillDraw(surface->surface_id());

[danakj, 2017/06/14 18:53:12]

Would it be better to collect a set of surface ids and give them all in a single
callback? Would that let the consumer do smarter things? Will each user of the
callback have to do to the same otherwise?

[gklassen, 2017/06/14 19:29:11]

In the consumer we are currently contemplating it needs to do the same work for
each surface ( move the associated hit test data to an active list ) - so it
doesn't seem like it would provide much immediate benefit but we may want to
revisit later and if we add other consumers.    

+  }
 
   CHECK(debug_weak_this.get());
   for (const auto& render_pass : frame.render_pass_list) {