Fix a crash on Windows when saving the stale content of an evicted app window

This is only supported on ChromeOS, so make it conditional and follow
the example of Browser::ShouldShowStaleContentOnEviction():
https://cs.chromium.org/chromium/src/chrome/browser/ui/browser.cc?dr=CSs&q=Browser::ShouldShowStaleContentOnEviction&g=0&l=1453

BUG=1036760

Change-Id: Idcf003483600046a4e64ebe0b681bbdb30b8449a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1992515Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#729809}

extensions/browser/app_window/app_window.cc

@@ -455,7 +455,11 @@ void AppWindow::ExitPictureInPicture() {
 }
 
 bool AppWindow::ShouldShowStaleContentOnEviction(content::WebContents* source) {
+#if defined(OS_CHROMEOS)
   return true;
+#else
+  return false;
+#endif  // defined(OS_CHROMEOS)
 }
 
 bool AppWindow::OnMessageReceived(const IPC::Message& message,