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, 2017/06/15 19:51:20]

Just realized there's no test that this is happening, which means it could
regress in the future. Can you add a test to show this is called for aggregated
surfaces, and not for ones that are skipped? LGTM with a test

[gklassen, 2017/06/15 20:00:21]

Good call, Thank you. Test coming soon... 

[gklassen, 2017/06/15 21:07:57]

Done.

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