cc: Make OutputSurfaceFrame::sub_buffer_rect optional

cc/output/gl_renderer.cc

Index: cc/output/gl_renderer.cc
diff --git a/cc/output/gl_renderer.cc b/cc/output/gl_renderer.cc
index e03abea9585917d6c56f7bae69771a829076e3a5..72a7dc7d2de2aa2315487cd14eaae2a5ad2defdf 100644
--- a/cc/output/gl_renderer.cc
+++ b/cc/output/gl_renderer.cc
@@ -2648,12 +2648,9 @@ void GLRenderer::SwapBuffers(std::vector<ui::LatencyInfo> latency_info) {
                   FlippedRootFramebuffer() ? flipped_y_pos_of_rect_bottom
                                            : swap_buffer_rect_.y(),
                   swap_buffer_rect_.width(), swap_buffer_rect_.height());
-  } else {
+  } else if (swap_buffer_rect_.IsEmpty() && allow_empty_swap_) {
     // Expand the swap rect to the full surface unless it's empty, and empty

[danakj, 2017/02/22 14:45:06]

Oh, this comment isn't quite right anymore. We're not expanding the rect so much
as neglecting to send one.

[halliwell, 2017/02/22 20:59:39]

Indeed, removed (not sure a comment is super necessary here any longer, but let
me know if you disagree).

     // swap is allowed.
-    if (!swap_buffer_rect_.IsEmpty() || !allow_empty_swap_) {
-      swap_buffer_rect_ = gfx::Rect(surface_size);
-    }
     output_frame.sub_buffer_rect = swap_buffer_rect_;
   }