servo: Merge #14436 - Make restyle tracking more granular (from bholley:granular_restyle); r=emilio

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.

Source-Repo: https://github.com/servo/servo
Source-Revision: b9a8ccd775c3192e3810a1730b1d0bc2b5c9dfb6

servo/components/layout/context.rs

@@ -18,6 +18,7 @@ use net_traits::image_cache_thread::{ImageCacheChan, ImageCacheThread, ImageResp
 use net_traits::image_cache_thread::{ImageOrMetadataAvailable, UsePlaceholder};
 use parking_lot::RwLock;
 use servo_url::ServoUrl;
+use std::borrow::Borrow;
 use std::cell::{RefCell, RefMut};
 use std::collections::HashMap;
 use std::hash::BuildHasherDefault;
@@ -88,6 +89,12 @@ pub struct SharedLayoutContext {
                                                   BuildHasherDefault<FnvHasher>>>>,
 }
 
+impl Borrow<SharedStyleContext> for SharedLayoutContext {
+    fn borrow(&self) -> &SharedStyleContext {
+        &self.style_context
+    }
+}
+
 pub struct LayoutContext<'a> {
     pub shared: &'a SharedLayoutContext,
     cached_local_layout_context: Rc<LocalLayoutContext>,