add setting to enable site isolation

[diracdeltas]

Test Plan: #12491

context: https://www.chromium.org/Home/chromium-security/ssca

We should have a setting to enable site isolation on all sites: https://www.chromium.org/Home/chromium-security/site-isolation. The memory footprint could be a concern, but the other known issues seem like relatively minor edge cases.

[diracdeltas]

decided to enable off-by-default at first due to high likelihood of breaking sites

[BrendanEich]

+1 on this. The hope is that with our by-default aggressive 3rd party blocking, the memory problems that Chrome is contending with won't be nearly as challenging.

[srirambv]

@diracdeltas should we add a info flag to the Site Isolation header linking the context?

[diracdeltas]

Weirdly, the site isolation setting doesn't seem to work in the 0.19.126 packaged build, but it does work when running the 0.19.126dev tag using 'npm start'. @bridiver any ideas what could cause this?
test steps:

1. load https://jsfiddle.net/avse30aj/
2. open devtools
3. make sure no requests to jshell.net appear in the devtools panel

[kjozwiak]

When viewing https://jsfiddle.net/avse30aj/ with site isolation enabled via about:preferences#security, you'll still see network requests from fiddle.jshell.net under the browser console, example:

• https://fiddle.jshell.net/js/lib/dummy.js
• https://fiddle.jshell.net/avse30aj/show/
• https://fiddle.jshell.net/js/_dist-editor.js?update_05_01_2018_1

@diracdeltas mentioned that this is working when running from source using npm but doesn't seem to be working when using the packaged 0.19.126 build.

Reproducible on all three platforms:

• macOS using 0.19.126 - reproduced by @LaurenWags and @diracdeltas
• Win 10 x64 using 0.19.126 - reproduced
• Ubuntu 17.10 x64 - reproduced

[bsclifton]

Fixed with #12518