cc: Partial draw without partial swap support.

cc/output/direct_renderer.cc

Index: cc/output/direct_renderer.cc
diff --git a/cc/output/direct_renderer.cc b/cc/output/direct_renderer.cc
index e9b62d29587bfdec92dd74dbb2f73f9aac87266f..78c34c84d10317fb6a65a1946fac09416c5989c6 100644
--- a/cc/output/direct_renderer.cc
+++ b/cc/output/direct_renderer.cc
@@ -337,15 +337,14 @@ void DirectRenderer::DrawFrame(RenderPassList* render_passes_in_draw_order,
         current_frame()->root_render_pass->output_rect;
   }
 
+  allow_partial_draw_ =
+      use_partial_swap_ ||
+      output_surface_->CanPartialDraw(current_frame()->root_damage_rect);
+
   // We can skip all drawing if the damage rect is now empty.
   bool skip_drawing_root_render_pass =
       current_frame()->root_damage_rect.IsEmpty() && allow_empty_swap_;
 
-  // If we have to draw but don't support partial swap, the whole output should
-  // be considered damaged.
-  if (!skip_drawing_root_render_pass && !use_partial_swap_)
-    current_frame()->root_damage_rect = root_render_pass->output_rect;
-
   if (skip_drawing_root_render_pass) {
     // If any of the overlays is the output surface, then ensure that the
     // backbuffer be allocated (allocation of the backbuffer is a side-effect
@@ -532,7 +531,7 @@ void DirectRenderer::DrawRenderPass(const RenderPass* render_pass) {
         DeviceViewportRectInDrawSpace());
   }
 
-  if (use_partial_swap_) {
+  if (allow_partial_draw_) {
     render_pass_scissor_in_draw_space.Intersect(
         current_frame()->ComputeScissorRectForRenderPass());
   }