cc: Change RTree::Search to return results, add comments, more tests.

cc/paint/display_item_list.cc

Index: cc/paint/display_item_list.cc
diff --git a/cc/paint/display_item_list.cc b/cc/paint/display_item_list.cc
index 88e11106bacfca1789eaefb3de1ed9f925ace7ac..efe5e6f3bc700a48a48da6db4868f4a6cc0b099d 100644
--- a/cc/paint/display_item_list.cc
+++ b/cc/paint/display_item_list.cc
@@ -214,8 +214,7 @@ void DisplayItemList::Raster(SkCanvas* canvas,
   if (!GetCanvasClipBounds(canvas, &canvas_playback_rect))
     return;
 
-  std::vector<size_t> indices;
-  rtree_.Search(canvas_playback_rect, &indices);
+  std::vector<size_t> indices = rtree_.Search(canvas_playback_rect);
   for (size_t i = 0; i < indices.size(); ++i) {
     // We use a callback during solid color analysis on the compositor thread to
     // break out early. Since we're handling a sequence of pictures via rtree