EXO: Add GetRootWindowForNewWindows function to wm_helper interface

Added GetRootWindowForNewWindows and implemented in wm_helper_chromeos.
This allows simple retrieval of root window from
window manager, used for setting parent window when creating
widgets (https://chromium-review.googlesource.com/c/chromium/src/+/1298172/20,
fullscreen_shell_surface.cc).

Bug: 896710
Test: Built and ran on ChromeOS
Change-Id: Ie43bea17c31cdf79402fadea441d5bc924e64503
Reviewed-on: https://chromium-review.googlesource.com/c/1302823Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Commit-Queue: Shawn Gallea <sagallea@google.com>
Cr-Commit-Position: refs/heads/master@{#605806}

components/exo/shell_surface_base.cc

@@ -960,7 +960,8 @@ void ShellSurfaceBase::CreateShellSurfaceWidget(
   params.parent =
       parent_ ? parent_
               : ash::Shell::GetContainer(
-                    ash::Shell::GetRootWindowForNewWindows(), container_);
+                    WMHelper::GetInstance()->GetRootWindowForNewWindows(),
+                    container_);
   params.bounds = gfx::Rect(origin_, gfx::Size());
   bool activatable = activatable_;
   // ShellSurfaces in system modal container are only activatable if input

components/exo/wm_helper.h

@@ -91,6 +91,7 @@ class WMHelper : public aura::client::DragDropDelegate {
   virtual aura::Window* GetPrimaryDisplayContainer(int container_id) = 0;
   virtual aura::Window* GetActiveWindow() const = 0;
   virtual aura::Window* GetFocusedWindow() const = 0;
+  virtual aura::Window* GetRootWindowForNewWindows() const = 0;
   virtual aura::client::CursorClient* GetCursorClient() = 0;
   virtual void AddPreTargetHandler(ui::EventHandler* handler) = 0;
   virtual void PrependPreTargetHandler(ui::EventHandler* handler) = 0;

components/exo/wm_helper_chromeos.cc

@@ -170,6 +170,10 @@ aura::Window* WMHelperChromeOS::GetFocusedWindow() const {
   return focus_client->GetFocusedWindow();
 }
 
+aura::Window* WMHelperChromeOS::GetRootWindowForNewWindows() const {
+  return ash::Shell::GetRootWindowForNewWindows();
+}
+
 aura::client::CursorClient* WMHelperChromeOS::GetCursorClient() {
   return aura::client::GetCursorClient(ash::Shell::GetPrimaryRootWindow());
 }

components/exo/wm_helper_chromeos.h

@@ -86,6 +86,7 @@ class WMHelperChromeOS : public WMHelper {
   aura::Window* GetPrimaryDisplayContainer(int container_id) override;
   aura::Window* GetActiveWindow() const override;
   aura::Window* GetFocusedWindow() const override;
+  aura::Window* GetRootWindowForNewWindows() const override;
   aura::client::CursorClient* GetCursorClient() override;
   void AddPreTargetHandler(ui::EventHandler* handler) override;
   void PrependPreTargetHandler(ui::EventHandler* handler) override;