style: Fix cascade order of !important in Shadow DOM.

No cleaner ideas right now that carrying that counter around... Maybe a custom
type may be cleaner?

This makes ApplicableDeclarationBlock a bit bigger. I could probably try to make
the counter a 4 / 5-bit number or something and pack the counter there in the
SourceOrderAndCascadeLevel somehow...

But doesn't seem really worth the churn, and can be done as a followup in any
case. Let me know if you want to block on that.

Bug: 1454162
Reviewed-by: heycam
MozReview-Commit-ID: 1LdW9S4xA6f

components/style/applicable_declarations.rs

@@ -5,7 +5,7 @@
 //! Applicable declarations management.
 
 use properties::PropertyDeclarationBlock;
-use rule_tree::{CascadeLevel, StyleSource};
+use rule_tree::{CascadeLevel, ShadowCascadeOrder, StyleSource};
 use servo_arc::Arc;
 use shared_lock::Locked;
 use smallvec::SmallVec;
@@ -83,6 +83,8 @@ pub struct ApplicableDeclarationBlock {
     order_and_level: SourceOrderAndCascadeLevel,
     /// The specificity of the selector this block is represented by.
     pub specificity: u32,
+    /// The order in the tree of trees we carry on.
+    pub shadow_cascade_order: ShadowCascadeOrder,
 }
 
 impl ApplicableDeclarationBlock {
@@ -97,16 +99,24 @@ impl ApplicableDeclarationBlock {
             source: StyleSource::Declarations(declarations),
             order_and_level: SourceOrderAndCascadeLevel::new(0, level),
             specificity: 0,
+            shadow_cascade_order: 0,
         }
     }
 
     /// Constructs an applicable declaration block from the given components
     #[inline]
-    pub fn new(source: StyleSource, order: u32, level: CascadeLevel, specificity: u32) -> Self {
+    pub fn new(
+        source: StyleSource,
+        order: u32,
+        level: CascadeLevel,
+        specificity: u32,
+        shadow_cascade_order: u32,
+    ) -> Self {
         ApplicableDeclarationBlock {
-            source: source,
+            source,
             order_and_level: SourceOrderAndCascadeLevel::new(order, level),
-            specificity: specificity,
+            specificity,
+            shadow_cascade_order,
         }
     }
 
@@ -122,11 +132,11 @@ impl ApplicableDeclarationBlock {
         self.order_and_level.level()
     }
 
-    /// Convenience method to consume self and return the source alongside the
-    /// level.
+    /// Convenience method to consume self and return the right thing for the
+    /// rule tree to iterate over.
     #[inline]
-    pub fn order_and_level(self) -> (StyleSource, CascadeLevel) {
+    pub fn for_rule_tree(self) -> (StyleSource, CascadeLevel, ShadowCascadeOrder) {
         let level = self.level();
-        (self.source, level)
+        (self.source, level, self.shadow_cascade_order)
     }
 }