Chrome for Android is only open source as of a few months ago. Before that, the UI was entirely proprietary.
I'd see the main general motivations for something like MicroG as being a sense of trust, privacy, control. As long as even 0.1% of any packaged end-product is closed source, this isn't possible.
From https://developer.chrome.com/multidevice/faq :
> Is Chrome for Android open source?
> Chrome for Android is derived from Chromium. Since the launch of the first version, we have steadily open sourced all the critical components. You can build various Chromium components for Android as used in Chrome for Android using the instructions here.
This is just a long way of saying "no"