LibWeb: Support simplest form of CSSStyleDeclaration.setProperty()

This patch adds the setProperty(name, value) API to CSSStyleDeclaration.
Setting an invalid or empty value will cause the property to be removed
from the declaration.

Note that this only works on mutable declarations (i.e element.style)
and not on resolved declarations (i.e window.getComputedStyle(element)).

Userland/Libraries/LibWeb/Bindings/CSSStyleDeclarationWrapperCustom.cpp

@@ -44,7 +44,8 @@ bool CSSStyleDeclarationWrapper::internal_set(JS::PropertyName const& name, JS::
     if (vm().exception())
         return false;
 
-    return impl().set_property(property_id, css_text);
+    impl().set_property(property_id, css_text);
+    return true;
 }
 
 }

Userland/Libraries/LibWeb/CSS/CSSStyleDeclaration.cpp

@@ -64,9 +64,12 @@ Optional<StyleProperty> PropertyOwningCSSStyleDeclaration::property(PropertyID p
 bool PropertyOwningCSSStyleDeclaration::set_property(PropertyID property_id, StringView css_text)
 {
     auto new_value = parse_css_value(CSS::ParsingContext {}, css_text, property_id);
-    // FIXME: What are we supposed to do if we can't parse it?
-    if (!new_value)
+    if (!new_value) {
+        m_properties.remove_all_matching([&](auto& entry) {
+            return entry.property_id == property_id;
+        });
         return false;
+    }
 
     ScopeGuard style_invalidation_guard = [&] {
         auto& declaration = verify_cast<CSS::ElementInlineCSSStyleDeclaration>(*this);
@@ -102,4 +105,12 @@ String CSSStyleDeclaration::get_property_value(StringView property_name) const
     return maybe_property->value->to_string();
 }
 
+void CSSStyleDeclaration::set_property(StringView property_name, StringView css_text)
+{
+    auto property_id = property_id_from_string(property_name);
+    if (property_id == CSS::PropertyID::Invalid)
+        return;
+    set_property(property_id, css_text);
+}
+
 }

Userland/Libraries/LibWeb/CSS/CSSStyleDeclaration.h

@@ -34,6 +34,8 @@ public:
     virtual Optional<StyleProperty> property(PropertyID) const = 0;
     virtual bool set_property(PropertyID, StringView css_text) = 0;
 
+    void set_property(StringView property_name, StringView css_text);
+
     String get_property_value(StringView property) const;
 
 protected:

Userland/Libraries/LibWeb/CSS/CSSStyleDeclaration.idl

@@ -6,4 +6,6 @@ interface CSSStyleDeclaration {
 
     CSSOMString getPropertyValue(CSSOMString property);
 
+    [CEReactions] undefined setProperty(CSSOMString property, [LegacyNullToEmptyString] CSSOMString value);
+
 };