style: Use enums for text-align / text-align-last.

This also fixes some backwards logic in nsBlockFrame::ReflowDirtyLines, and adds
some static assertions to nsGenericHTMLElement that almost cause a very subtle
bug.

Depends on D63792

Differential Revision: https://phabricator.services.mozilla.com/D63793

components/style/values/specified/text.rs

@@ -517,6 +517,35 @@ impl ToCss for TextTransformOther {
     }
 }
 
+/// Specified and computed value of text-align-last.
+#[derive(
+    Clone,
+    Copy,
+    Debug,
+    Eq,
+    FromPrimitive,
+    Hash,
+    MallocSizeOf,
+    Parse,
+    PartialEq,
+    SpecifiedValueInfo,
+    ToComputedValue,
+    ToCss,
+    ToResolvedValue,
+    ToShmem,
+)]
+#[allow(missing_docs)]
+#[repr(u8)]
+pub enum TextAlignLast {
+    Auto,
+    Start,
+    End,
+    Left,
+    Right,
+    Center,
+    Justify,
+}
+
 /// Specified value of text-align keyword value.
 #[derive(
     Clone,
@@ -535,14 +564,18 @@ impl ToCss for TextTransformOther {
     ToShmem,
 )]
 #[allow(missing_docs)]
+#[repr(u8)]
 pub enum TextAlignKeyword {
     Start,
-    End,
     Left,
     Right,
     Center,
     #[cfg(any(feature = "gecko", feature = "servo-layout-2013"))]
     Justify,
+    #[css(skip)]
+    #[cfg(feature = "gecko")]
+    Char,
+    End,
     #[cfg(feature = "gecko")]
     MozCenter,
     #[cfg(feature = "gecko")]
@@ -555,9 +588,6 @@ pub enum TextAlignKeyword {
     ServoLeft,
     #[cfg(feature = "servo-layout-2013")]
     ServoRight,
-    #[css(skip)]
-    #[cfg(feature = "gecko")]
-    Char,
 }
 
 /// Specified value of text-align property.
@@ -579,14 +609,6 @@ pub enum TextAlign {
     MozCenterOrInherit,
 }
 
-impl TextAlign {
-    /// Convert an enumerated value coming from Gecko to a `TextAlign`.
-    #[cfg(feature = "gecko")]
-    pub fn from_gecko_keyword(kw: u32) -> Self {
-        TextAlign::Keyword(TextAlignKeyword::from_gecko_keyword(kw))
-    }
-}
-
 impl ToComputedValue for TextAlign {
     type ComputedValue = TextAlignKeyword;