Leave fullscreen mode in an app window when ESC key is pressed.

apps/shell_window.cc

Index: apps/shell_window.cc
diff --git a/apps/shell_window.cc b/apps/shell_window.cc
index 5e4098f504d51f62a3971a450f99b155572babed..cc228873a2d35acf195f26ac4347121b13edd176 100644
--- a/apps/shell_window.cc
+++ b/apps/shell_window.cc
@@ -286,6 +286,13 @@ void ShellWindow::AddNewContents(WebContents* source,
 void ShellWindow::HandleKeyboardEvent(
     WebContents* source,
     const content::NativeWebKeyboardEvent& event) {
+  // If the window is currently fullscreen, ESC should leave fullscreen.
+  if (event.windowsKeyCode == ui::VKEY_ESCAPE &&
+      (fullscreen_for_tab_ || fullscreen_for_window_api_)) {
+    Restore();
+    return;
+  }
+
   native_app_window_->HandleKeyboardEvent(event);
 }