LibWeb: Support '<zero>' in '<color-stop-angle>`

`<color-stop-angle> = [ <angle-percentage> | <zero> ]{1,2}` but we were
previously parsing instead as `<angle-percentage>{1,2}`.

Author: Calme1709

Libraries/LibWeb/CSS/Parser/GradientParsing.cpp

@@ -126,7 +126,18 @@ Optional<Vector<AngularColorStopListElement>> Parser::parse_angular_color_stop_l
     //   <angular-color-stop> , [ <angular-color-hint>? , <angular-color-stop> ]#
     return parse_color_stop_list<AngularColorStopListElement>(
         tokens,
-        [&](auto& it) { return parse_angle_percentage(it); });
+        [&](TokenStream<ComponentValue>& it) -> Optional<AnglePercentage> {
+            if (tokens.next_token().is(Token::Type::Number)) {
+                auto transaction = tokens.begin_transaction();
+                auto numeric_value = tokens.consume_a_token().token().number_value();
+                if (numeric_value == 0) {
+                    transaction.commit();
+                    return Angle::make_degrees(0);
+                }
+            }
+
+            return parse_angle_percentage(it);
+        });
 }
 
 Optional<InterpolationMethod> Parser::parse_interpolation_method(TokenStream<ComponentValue>& tokens)

Tests/LibWeb/Text/expected/wpt-import/css/css-images/gradient/color-stops-parsing.txt

@@ -2,8 +2,7 @@ Harness status: OK
 
 Found 162 tests
 
-160 Pass
-2 Fail
+162 Pass
 Pass	linear-gradient() [ unparsable ]
 Pass	linear-gradient(black, 25%) [ unparsable ]
 Pass	linear-gradient(black, invalid) [ unparsable ]
@@ -125,7 +124,7 @@ Pass	conic-gradient(0%, black, white) [ unparsable ]
 Pass	conic-gradient(black) [ parsable ]
 Pass	conic-gradient(black 0%) [ parsable ]
 Pass	conic-gradient(black, white) [ parsable ]
-Fail	conic-gradient(black 0, white) [ parsable ]
+Pass	conic-gradient(black 0, white) [ parsable ]
 Pass	conic-gradient(black 0%, white) [ parsable ]
 Pass	conic-gradient(black 0%, white 100%) [ parsable ]
 Pass	conic-gradient(black, green, white) [ parsable ]
@@ -152,7 +151,7 @@ Pass	repeating-conic-gradient(0%, black, white) [ unparsable ]
 Pass	repeating-conic-gradient(black) [ parsable ]
 Pass	repeating-conic-gradient(black 0%) [ parsable ]
 Pass	repeating-conic-gradient(black, white) [ parsable ]
-Fail	repeating-conic-gradient(black 0, white) [ parsable ]
+Pass	repeating-conic-gradient(black 0, white) [ parsable ]
 Pass	repeating-conic-gradient(black 0%, white) [ parsable ]
 Pass	repeating-conic-gradient(black 0%, white 100%) [ parsable ]
 Pass	repeating-conic-gradient(black, green, white) [ parsable ]