gpu: Empty swaps for surfaceless output surfaces.

cc/output/direct_renderer.h

Index: cc/output/direct_renderer.h
diff --git a/cc/output/direct_renderer.h b/cc/output/direct_renderer.h
index 96e66133c2019fc156bbaa4ec8d8b21058c133cf..d3423c96478ab1e3c235b45827bbe475976dee67 100644
--- a/cc/output/direct_renderer.h
+++ b/cc/output/direct_renderer.h
@@ -188,6 +188,8 @@ class CC_EXPORT DirectRenderer {
   bool allow_empty_swap_ = false;
   // Whether partial swap can be used.
   bool use_partial_swap_ = false;

[Daniele Castagna, 2017/04/19 19:10:36]

This names are a little bit confusing, or at least, it seems like a look at the
code is needed to clear out what they do.

Can we rename them?

Maybe explicitly mentioning if post_sub_buffers is used or not.

[reveman, 2017/04/19 23:59:03]

Sure. Either use_post_sub_buffer or use_sub_buffer_rect instead of
use_partial_swap might make this easier to read. I'll do that if other reviewers
agree.

+  // Whether non-empty partial swap should be allowed.
+  bool allow_partial_swap_ = false;
   // Whether overdraw feedback is enabled and can be used.
   bool overdraw_feedback_ = false;
   // Whether the SetDrawRectangle and EnableDCLayers commands are in