cc: Implement overdraw feedback debugging feature.

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 d0798231bcb21092967c4ca6161b0881e6b54360..81fc3b85374ba7eb6332bffd42af6d73b44e82ea 100644
--- a/cc/output/direct_renderer.cc
+++ b/cc/output/direct_renderer.cc
@@ -87,6 +87,10 @@ void DirectRenderer::Initialize() {
       context_provider->ContextCapabilities().commit_overlay_planes)
     allow_empty_swap_ = true;
 
+  DLOG_IF(WARNING, settings_->show_overdraw_feedback &&
+                       !output_surface_->capabilities().supports_stencil)
+      << "Overdraw feedback enabled on platform without support.";
+
   initialized_ = true;
 }
 
@@ -225,6 +229,14 @@ void DirectRenderer::DrawFrame(RenderPassList* render_passes_in_draw_order,
   RenderPass* root_render_pass = render_passes_in_draw_order->back().get();
   DCHECK(root_render_pass);
 
+  bool overdraw_tracing_enabled;
+  TRACE_EVENT_CATEGORY_GROUP_ENABLED(
+      TRACE_DISABLED_BY_DEFAULT("cc.debug.overdraw"),
+      &overdraw_tracing_enabled);
+  overdraw_feedback_ =
+      (settings_->show_overdraw_feedback || overdraw_tracing_enabled) &&
+      output_surface_->capabilities().supports_stencil;
+
   DrawingFrame frame;
   frame.render_passes_in_draw_order = render_passes_in_draw_order;
   frame.root_render_pass = root_render_pass;
@@ -238,17 +250,20 @@ void DirectRenderer::DrawFrame(RenderPassList* render_passes_in_draw_order,
   // can leave the window at the wrong size if we never draw and the proper
   // viewport size is never set.
   bool frame_has_alpha = frame.root_render_pass->has_transparent_background;
+  bool use_stencil = overdraw_feedback_;
   if (device_viewport_size != reshape_surface_size_ ||
       device_scale_factor != reshape_device_scale_factor_ ||
       device_color_space != reshape_device_color_space_ ||
-      frame_has_alpha != reshape_has_alpha_) {
+      frame_has_alpha != reshape_has_alpha_ ||
+      use_stencil != reshape_use_stencil_) {
     reshape_surface_size_ = device_viewport_size;
     reshape_device_scale_factor_ = device_scale_factor;
     reshape_device_color_space_ = device_color_space;
     reshape_has_alpha_ = frame.root_render_pass->has_transparent_background;
-    output_surface_->Reshape(reshape_surface_size_,
-                             reshape_device_scale_factor_,
-                             reshape_device_color_space_, reshape_has_alpha_);
+    reshape_use_stencil_ = overdraw_feedback_;
+    output_surface_->Reshape(
+        reshape_surface_size_, reshape_device_scale_factor_,
+        reshape_device_color_space_, reshape_has_alpha_, reshape_use_stencil_);
   }
 
   BeginDrawingFrame(&frame);