Use MD-ash's auto hide behavior for arc++ windows.

components/exo/shell_surface.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/exo/shell_surface.h"
 
 #include "ash/aura/wm_window_aura.h"
 #include "ash/common/shell_window_ids.h"
 #include "ash/common/wm/window_resizer.h"
 #include "ash/common/wm/window_state.h"
@@ skipping 1237 matching lines @@
       // Ensure the background area inside the shadow is solid black.
       // Clients that provide translucent contents should not be using
       // rectangular shadows as this method requires opaque contents to
       // cast a shadow that represent it correctly.
       shadow_underlay_->Init(ui::LAYER_SOLID_COLOR);
       shadow_underlay_->layer()->SetColor(SK_ColorBLACK);
       DCHECK(shadow_underlay_->layer()->fills_bounds_opaquely());
       window->AddChild(shadow_underlay_);
       window->StackChildAtBottom(shadow_underlay_);
     }
-    shadow_underlay_->layer()->SetOpacity(
-        rectangular_shadow_background_opacity_);
-    shadow_underlay_->SetBounds(shadow_bounds);
+
+    // Put the black background layer behind the window if
+    // 1) the window is in immersive fullscreen.
+    // 2) the window can control the bounds of the window in fullscreen (
+    //    thus the background can be visible).
+    // 3) the window has no transform (the transformed background may
+    //    not cover the entire background, e.g. overview mode).
+    if (widget_->IsFullscreen() &&
+        ash::wm::GetWindowState(window)->allow_set_bounds_in_maximized() &&
+        window->layer()->transform().IsIdentity()) {
+      gfx::Point origin;
+      origin -= window->bounds().origin().OffsetFromOrigin();
+      gfx::Rect background_bounds(origin, window->parent()->bounds().size());
+      shadow_underlay_->SetBounds(background_bounds);
+      shadow_underlay_->layer()->SetOpacity(1.f);
+    } else {
+      shadow_underlay_->SetBounds(shadow_bounds);
+      shadow_underlay_->layer()->SetOpacity(
+          rectangular_shadow_background_opacity_);
+    }
     shadow_underlay_->Show();
 
     wm::Shadow* shadow = wm::ShadowController::GetShadowForWindow(window);
     // Maximized/Fullscreen window does not create a shadow.
     if (!shadow)
       return;
 
     if (!shadow_overlay_) {
       shadow_overlay_ = new aura::Window(nullptr);
       DCHECK(shadow_overlay_->owned_by_parent());
       shadow_overlay_->set_ignore_events(true);
       shadow_overlay_->Init(ui::LAYER_NOT_DRAWN);
       shadow_overlay_->layer()->Add(shadow->layer());
       window->AddChild(shadow_overlay_);
       shadow_overlay_->Show();
     }
     shadow_overlay_->SetBounds(shadow_bounds);
     shadow->SetContentBounds(gfx::Rect(shadow_bounds.size()));
   }
 }
 
 }  // namespace exo