DevTools: [SSP] treat new rules out of style cascade.
Currently, as a new rule gets added with "+" button in StylesSidebarPane,
its properties are treated as a part of a style cascade. This results
in some of them being crossed-out, and some of them overriding
properties from other styles.
However, this is almost always wrong, as it doesn't respect rule's
This patch pulls new rule out of styles cascade, so its properties are
never crossed-out and it never overrides other style's properties.