DirectRenderer allows empty swap rects for CommitOverlayPlanes

cc/output/renderer.h

Index: cc/output/renderer.h
diff --git a/cc/output/renderer.h b/cc/output/renderer.h
index 82b3ae307c0b1712455b2935845a04c34db91f8a..f91b039e673422f3e11087abbfe7517b0eaccc8f 100644
--- a/cc/output/renderer.h
+++ b/cc/output/renderer.h
@@ -34,6 +34,9 @@ struct RendererCapabilitiesImpl {
 
   // Capabilities used on compositor thread only.
   bool using_partial_swap;
+  // Whether it's valid to SwapBuffers with an empty rect. Trivially true when
+  // |using_partial_swap|.
+  bool allow_empty_swap;
   bool using_egl_image;
   bool using_image;
   bool using_discard_framebuffer;