Auto merge of #14848 - bzbarsky:initial-styles, r=bholley

Stop using global initial styles for stylo; the initial styles need to be per-document

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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 https://bugzilla.mozilla.org/show_bug.cgi?id=1298588

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests on the servo side because behavior is unchanged.  Gecko-side tests probably exist.

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tests/unit/style/parsing/image.rs

@@ -8,6 +8,7 @@ use euclid::size::Size2D;
 use media_queries::CSSErrorReporterTest;
 use std::f32::consts::PI;
 use style::parser::ParserContext;
+use style::properties::ComputedValues;
 use style::stylesheets::Origin;
 use style::values::computed;
 use style::values::computed::{Angle, Context, ToComputedValue};
@@ -43,7 +44,14 @@ fn test_linear_gradient() {
     // Note that Angle(PI) is correct for top-to-bottom rendering, whereas Angle(0) would render bottom-to-top.
     // ref: https://developer.mozilla.org/en-US/docs/Web/CSS/angle
     let container = Size2D::new(Au::default(), Au::default());
-    let specified_context = Context::initial(container, true);
+    let initial_style = ComputedValues::initial_values();
+    let specified_context = Context {
+        is_root_element: true,
+        viewport_size: container,
+        inherited_style: initial_style,
+        style: initial_style.clone(),
+        font_metrics_provider: None,
+    };
     assert_eq!(specified::AngleOrCorner::None.to_computed_value(&specified_context),
                computed::AngleOrCorner::Angle(Angle(PI)));
 }