Auto merge of #18321 - canaltinova:default-linear-gradient, r=Manishearth

Properly set default direction of prefixed linear gradients

The default linear gradient direction is `to bottom`. This correspondes to
`top` keyword in prefixed linear gradients. We should preserve the default value.

---
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix [Bug 1395189](https://bugzilla.mozilla.org/show_bug.cgi?id=1395189)

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components/style/values/specified/image.rs

@@ -526,7 +526,10 @@ impl GradientKind {
             input.expect_comma()?;
             d
         } else {
-            LineDirection::Vertical(Y::Bottom)
+            match *compat_mode {
+                CompatMode::Modern => LineDirection::Vertical(Y::Bottom),
+                _ => LineDirection::Vertical(Y::Top),
+            }
         };
         Ok(GenericGradientKind::Linear(direction))
     }
@@ -615,10 +618,13 @@ impl GradientKind {
 }
 
 impl GenericsLineDirection for LineDirection {
-    fn points_downwards(&self) -> bool {
+    fn points_downwards(&self, compat_mode: CompatMode) -> bool {
         match *self {
             LineDirection::Angle(ref angle) => angle.radians() == PI,
-            LineDirection::Vertical(Y::Bottom) => true,
+            LineDirection::Vertical(Y::Bottom)
+                if compat_mode == CompatMode::Modern => true,
+            LineDirection::Vertical(Y::Top)
+                if compat_mode != CompatMode::Modern => true,
             _ => false,
         }
     }