disable chrome.webstore.install for inline extensions

[diracdeltas]

context: https://blog.chromium.org/2018/06/improving-extension-transparency-for.html

inline extensions are a huge malware risk. we should disable them in Brave.

AFAIK it is sufficient to disable the chrome.webstore.install method

note that in c71, chrome will disable it as well (possibly with a redirect to CWS which would be slightly better UX than silently failing).

[bbondy]

Noting possibly the right thing is to redirect instead of remove the API.

[bbondy]

@AilinLiao @bsclifton after this lands you'll have to rebase your patch I think.