Add support for the parent element (what is passed to Terminal.open)
being in a different (same origin) window. Accesses to DOM APIs such
as requestAnimationFrame and devicePixelRatio thus need to be scoped
to the corrent window, instead of assuming that it's the same as the
window/global scope where the code is running.
This is done by inferring a parent window at the creation time, and
then storing it in CoreBrowserService, which is already passed to most
places that need it.
To catch future regressions, an ESLint rule that checks for global
accesses is added (it uses AST selectors via the no-restricted-syntax
rule).
This should also be applicable when the parent element is an iframe.
Fixes#3758
Daniel Imms
jerch
Mihai Parparita