Use even fewer bits for source order and shrink ApplicableDeclarationsBlock by another word.

MozReview-Commit-ID: 7B1i1g0HLTj

components/style/rule_tree/mod.rs

@@ -12,6 +12,7 @@ use properties::{AnimationRules, Importance, LonghandIdSet, PropertyDeclarationB
 use shared_lock::{Locked, StylesheetGuards, SharedRwLockReadGuard};
 use smallvec::SmallVec;
 use std::io::{self, Write};
+use std::mem;
 use std::ptr;
 use std::sync::atomic::{AtomicPtr, AtomicUsize, Ordering};
 use stylearc::{Arc, NonZeroPtrMut};
@@ -229,7 +230,7 @@ impl RuleTree {
                              guards: &StylesheetGuards)
                              -> StrongRuleNode
     {
-        let rules = applicable_declarations.drain().map(|d| (d.source, d.level));
+        let rules = applicable_declarations.drain().map(|d| d.source_and_level());
         let rule_node = self.insert_ordered_rules_with_important(rules, guards);
         rule_node
     }
@@ -425,10 +426,18 @@ pub enum CascadeLevel {
     /// User-agent important rules.
     UAImportant,
     /// Transitions
+    ///
+    /// NB: If this changes from being last, change from_byte below.
     Transitions,
 }
 
 impl CascadeLevel {
+    /// Converts a raw byte to a CascadeLevel.
+    pub unsafe fn from_byte(byte: u8) -> Self {
+        debug_assert!(byte <= CascadeLevel::Transitions as u8);
+        mem::transmute(byte)
+    }
+
     /// Select a lock guard for this level
     pub fn guard<'a>(&self, guards: &'a StylesheetGuards<'a>) -> &'a SharedRwLockReadGuard<'a> {
         match *self {