Intent to prototype: CSS page-size

[Emily McDonough]

Summary: It is desirable for some web pages to specify the preferred media size to use when printing. The size property in @page rules provides this capability.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=851441

Standard: https://drafts.csswg.org/css-page-3/
Platform coverage: All
Preference: layout.css.page-size.enabled
DevTools bug: Currently there is no provision to debug or inspect page properties in general.
Other browsers: Supported in Blink since Chrome 15. Not supported in WebKit.

web-platform-tests: https://github.com/web-platform-tests/wpt/tree/master/css/css-page