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.

[bbondy]

QA note: zenhub has an extension on their page which uses this API.

[btlechowski]

Verification passes on

Brave	0.55.6 Chromium: 70.0.3538.16 (Official Build) dev (64-bit)
Revision	16ed95b41bb05e565b11fb66ac33c660b721f778-refs/branch-heads/3538@{#306}
OS	Windows 7

redirected to ZenHub for GitHub on chrome web store

Verification Passed on

Brave	0.55.6 Chromium: 70.0.3538.16 (Official Build) dev (64-bit)
Revision	16ed95b41bb05e565b11fb66ac33c660b721f778-refs/branch-heads/3538@{#306}
OS	Linux

• Verified installing Zenhub from their website redirects to CWS on a new tab and installs the extension

Went through verification using the following build under macOS 10.13.6 x64 - PASSED

Brave	0.55.6 Chromium: 70.0.3538.16 (Official Build) dev (64-bit)
Revision	16ed95b41bb05e565b11fb66ac33c660b721f778-refs/branch-heads/3538@{#306}
OS	Mac OS X

• Verified that installing via https://www.zenhub.com/extension redirects to CWS in a new tab and installs the extension via CWS and not the website.