Problem:
Chrome for iOS implements WebUI by fetching main html document and its subresources from net stack, flattening html and force feeding it to WKWebView. This approach uses simple html parser, regexps and large strings concatenation all because iOS 10 and lower does not support custom url protocols for WKWebView. Current solution works fine, but the code is quite complex 

Solution:
Starting from ios 11 WKWebView supports custom URL protocols via WKURLSchemeHandler API. There are no visible user benefits in using new API, and because Chrome has to support iOS10, there is no reason to switch. WKURLSchemeHandler works just fine as demonstrated in https://chromium-review.googlesource.com/c/566091/ and we should switch to use new API after dropping iOS 10.

Design doc:
https://docs.google.com/document/d/1vZFxG8ml2yKanIEzMEEosXJDGKfJRUDwlkz5qKkR1w8/edit#