Chrome Version       : 63.0.3213.3

ExclusiveAccessBubbleViews does

  // Create the contents view.
  ui::Accelerator accelerator(ui::VKEY_UNKNOWN, ui::EF_NONE);
  bool got_accelerator =
      bubble_view_context_->GetAcceleratorProvider()
          ->GetAcceleratorForCommandId(IDC_FULLSCREEN, &accelerator);
  DCHECK(got_accelerator);
  view_ = new SubtleNotificationView(this);
  browser_fullscreen_exit_accelerator_ = accelerator.GetShortcutText();


So it requires 
 - GetAcceleratorProvider() to return non-null, and
 - that provider to provide an accelerator to populate |browser_fullscreen_exit_accelerator_|


But later on, |browser_fullscreen_exit_accelerator_| may be ignored:

  if (bubble_type ==
          EXCLUSIVE_ACCESS_BUBBLE_TYPE_BROWSER_FULLSCREEN_EXIT_INSTRUCTION ||
      bubble_type ==
          EXCLUSIVE_ACCESS_BUBBLE_TYPE_EXTENSION_FULLSCREEN_EXIT_INSTRUCTION) {
    accelerator = browser_fullscreen_exit_accelerator_;
  } else {
    accelerator = l10n_util::GetStringUTF16(IDS_APP_ESC_KEY);
    if (bubble_type !=
        EXCLUSIVE_ACCESS_BUBBLE_TYPE_FULLSCREEN_EXIT_INSTRUCTION) {
      link_visible = false;
    }
  }

i.e., it's only used for browser/extension fullscreen, but not OS/immersive fullscreen modes, which just clobber |browser_fullscreen_exit_accelerator_| with IDS_APP_ESC_KEY

This came up in https://chromium-review.googlesource.com/c/chromium/src/+/648985/13/chrome/browser/ui/views/apps/chrome_native_app_window_views_aura_ash.cc#462


(there's probably a lot of cleanups around these bubbles once we can ditch the non-simplified codepaths and the chrome://flag).