Move to_bidi_level method from Stylo to Servo (#37343)

This method is only used in Servo and is the only reason that `stylo`
depends on `unicode-bidi`.

Stylo PR: https://github.com/servo/stylo/pull/196

components/layout/construct_modern.rs

@@ -21,7 +21,7 @@ use crate::formatting_contexts::{
     IndependentNonReplacedContents,
 };
 use crate::layout_box_base::LayoutBoxBase;
-use crate::style_ext::DisplayGeneratingBox;
+use crate::style_ext::{ComputedValuesExt, DisplayGeneratingBox};
 
 /// A builder used for both flex and grid containers.
 pub(crate) struct ModernContainerBuilder<'a, 'dom> {
@@ -152,7 +152,7 @@ impl<'a, 'dom> ModernContainerBuilder<'a, 'dom> {
                         self.context,
                         true,  /* has_first_formatted_line */
                         false, /* is_single_line_text_box */
-                        self.info.style.writing_mode.to_bidi_level(),
+                        self.info.style.to_bidi_level(),
                     )?;
 
                     let block_formatting_context = BlockFormattingContext::from_block_container(

components/layout/flow/construct.rs

@@ -230,7 +230,7 @@ impl<'dom, 'style> BlockContainerBuilder<'dom, 'style> {
             self.context,
             !self.have_already_seen_first_line_for_text_indent,
             self.info.is_single_line_text_input(),
-            self.info.style.writing_mode.to_bidi_level(),
+            self.info.style.to_bidi_level(),
         )
     }
 
@@ -539,7 +539,7 @@ impl<'dom> BlockContainerBuilder<'dom, '_> {
                 builder.split_around_block_and_finish(
                     self.context,
                     !self.have_already_seen_first_line_for_text_indent,
-                    self.info.style.writing_mode.to_bidi_level(),
+                    self.info.style.to_bidi_level(),
                 )
             })
         {

components/layout/style_ext.rs

@@ -28,6 +28,7 @@ use style::values::generics::position::{GenericAspectRatio, PreferredRatio};
 use style::values::generics::transform::{GenericRotate, GenericScale, GenericTranslate};
 use style::values::specified::align::AlignFlags;
 use style::values::specified::{Overflow, WillChangeBits, box_ as stylo};
+use unicode_bidi::Level;
 use webrender_api as wr;
 use webrender_api::units::LayoutTransform;
 
@@ -365,6 +366,7 @@ pub(crate) trait ComputedValuesExt {
     ) -> bool;
     fn is_inline_box(&self, fragment_flags: FragmentFlags) -> bool;
     fn overflow_direction(&self) -> OverflowDirection;
+    fn to_bidi_level(&self) -> Level;
 }
 
 impl ComputedValuesExt for ComputedValues {
@@ -1017,6 +1019,17 @@ impl ComputedValuesExt for ComputedValues {
             downward,
         }
     }
+
+    /// The default bidirectional embedding level for the writing mode of this style.
+    ///
+    /// Returns bidi level 0 if the mode is LTR, or 1 otherwise.
+    fn to_bidi_level(&self) -> Level {
+        if self.writing_mode.is_bidi_ltr() {
+            Level::ltr()
+        } else {
+            Level::rtl()
+        }
+    }
 }
 
 pub(crate) enum LayoutStyle<'a> {