Make Restyle tracking more granular.

The primary idea of this patch is to ditch the rigid enum of Previous/Current
styles, and replace it with a series of indicators for the various types of
work that needs to be performed (expanding snapshots, rematching, recascading,
and damage processing). This loses us a little bit of sanity checking (since
the up-to-date-ness of our style is no longer baked into the type system), but
gives us a lot more flexibility that we'll need going forward (especially when
we separate matching from cascading). We also eliminate get_styling_mode in
favor of a method on the traversal.

This patch does a few other things as ridealongs:
* Temporarily eliminates the handling for transfering ownership of styles to the
  frame. We'll need this again at some point, but for now it's causing too much
  complexity for a half-implemented feature.
* Ditches TRestyleDamage, which is no longer necessary post-crate-merge, and is
  a constant source of compilation failures from either needing to be imported
  or being unnecessarily imported (which varies between gecko and servo).
* Expands Snapshots for the traversal root, which was missing before.
* Fixes up the skip_root stuff to avoid visiting the skipped root.
* Unifies parallel traversal and avoids spawning for a single work item.
* Adds an explicit pre_traverse step do any pre-processing and determine whether
  we need to traverse at all.

MozReview-Commit-ID: IKhLAkAigXE

components/style/context.rs

@@ -38,14 +38,6 @@ pub struct SharedStyleContext {
     /// Screen sized changed?
     pub screen_size_changed: bool,
 
-    /// Skip the root during traversal?
-    ///
-    /// This is used in Gecko to style newly-appended children without restyling
-    /// the parent. It would be cleaner to add an API to allow us to enqueue the
-    /// children directly from glue.rs.
-    #[cfg(feature = "gecko")]
-    pub skip_root: bool,
-
     /// The CSS selector stylist.
     pub stylist: Arc<Stylist>,