As the WebDialog / Widget API stands now, it doesn't seem possible to fix both this issue and crbug.com/884089 (this fix for this causes crbug.com/884089). See crbug.com/884089 for more context.

Rewording https://bugs.chromium.org/p/chromium/issues/detail?id=884089#c11:

The X on the top right of the dialog (the "close" button) still needs to be removed. This can be done by passing minimal_style==true to ShowWebDialogInContainer() [1], but this unfortunately messes up the change made in crbug.com/884089 (fullscreen): minimal_style marks the dialog as TYPE_WINDOW_FRAMELESS [2], so that when the dialog is rendered in views::Widget, it misses this logic which maximizes windows in tablet mode: [3]. 

In short, there is no way right now to both resolve this bug and and maximize the dialog in tablet mode.

Next action that needs to be taken on this bug: reach out to Chrome UI folks and propose a change to how TYPE_WINDOW_FRAMELESS is treated in tablet mode.

1) https://cs.chromium.org/chromium/src/chrome/browser/ui/browser_dialogs.h?q=browser_dialogs.h&sq=package:chromium&dr=CSs&l=99
2) https://cs.chromium.org/chromium/src/chrome/browser/ui/views/chrome_web_dialog_view.cc?q=ShowWebDialogInContainer&sq=package:chromium&g=0&l=62
3) https://cs.chromium.org/chromium/src/ui/views/widget/widget.cc?rcl=67c4f9f83057617d79a2e993dc8e2998de875880&l=373