[compositing] isolation property should be renamed

If all the isolation property does is create a stacking context [1][2] then
it seems like it should be called stacking-context: true to reveal it's
purpose, otherwise we're just going to have blog posts about the "secret
css hacks" to create stacking contexts using isolation: isolate as stacking
contexts have all kinds of other side effects.

The property also does not seem to be specific to blending, and the
isolation naming is confusing given that there's talk of layout/style
isolation, bidi isolation, and now blend isolation.

See also the discussion from blink-dev@ [3].

[1] http://dev.w3.org/fxtf/compositing-1/#isolation
[2] https://developer.mozilla.org/en-US/docs/Web/CSS/isolation
[3]
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/WoLwgoPB-GE

- E