Bug 1208951 - Part 4: Add an nsCSSParser entry point to parse a single longhand property and return an nsCSSValue. r=birtles

It is a slight hassle for code to have to create a Declaration object to
pass in to nsCSSParser::ParseProperty when all it wants is the resulting
single nsCSSValue, if parsing a longhand property.  Declaration is also
inaccessible outside layout/style/.

layout/style/nsCSSParser.cpp

@@ -168,6 +168,12 @@ public:
                      bool* aChanged,
                      bool aIsImportant,
                      bool aIsSVGMode);
+  void ParseLonghandProperty(const nsCSSProperty aPropID,
+                             const nsAString& aPropValue,
+                             nsIURI* aSheetURL,
+                             nsIURI* aBaseURL,
+                             nsIPrincipal* aSheetPrincipal,
+                             nsCSSValue& aValue);
 
   void ParseMediaList(const nsSubstring& aBuffer,
                       nsIURI* aURL, // for error reporting
@@ -1756,6 +1762,33 @@ CSSParserImpl::ParseRule(const nsAString&        aRule,
   return rv;
 }
 
+void
+CSSParserImpl::ParseLonghandProperty(const nsCSSProperty aPropID,
+                                     const nsAString& aPropValue,
+                                     nsIURI* aSheetURL,
+                                     nsIURI* aBaseURL,
+                                     nsIPrincipal* aSheetPrincipal,
+                                     nsCSSValue& aValue)
+{
+  MOZ_ASSERT(aPropID < eCSSProperty_COUNT_no_shorthands,
+             "ParseLonghandProperty must only take a longhand property");
+
+  Declaration declaration;
+  declaration.InitializeEmpty();
+
+  bool changed;
+  ParseProperty(aPropID, aPropValue, aSheetURL, aBaseURL, aSheetPrincipal,
+                &declaration, &changed,
+                /* aIsImportant */ false,
+                /* aIsSVGMode */ false);
+
+  if (changed) {
+    aValue = *declaration.GetNormalBlock()->ValueFor(aPropID);
+  } else {
+    aValue.Reset();
+  }
+}
+
 void
 CSSParserImpl::ParseProperty(const nsCSSProperty aPropID,
                              const nsAString& aPropValue,
@@ -16096,6 +16129,19 @@ nsCSSParser::ParseProperty(const nsCSSProperty aPropID,
                   aIsImportant, aIsSVGMode);
 }
 
+void
+nsCSSParser::ParseLonghandProperty(const nsCSSProperty aPropID,
+                                   const nsAString&    aPropValue,
+                                   nsIURI*             aSheetURI,
+                                   nsIURI*             aBaseURI,
+                                   nsIPrincipal*       aSheetPrincipal,
+                                   nsCSSValue&         aResult)
+{
+  static_cast<CSSParserImpl*>(mImpl)->
+    ParseLonghandProperty(aPropID, aPropValue, aSheetURI, aBaseURI,
+                          aSheetPrincipal, aResult);
+}
+
 void
 nsCSSParser::ParseVariable(const nsAString&    aVariableName,
                            const nsAString&    aPropValue,

layout/style/nsCSSParser.h

@@ -135,6 +135,16 @@ public:
                      bool                aIsImportant,
                      bool                aIsSVGMode = false);
 
+  // Same as ParseProperty but returns an nsCSSValue in aResult
+  // rather than storing the property in a Declaration.  aPropID
+  // must be a longhand property.
+  void ParseLonghandProperty(const nsCSSProperty aPropID,
+                             const nsAString&    aPropValue,
+                             nsIURI*             aSheetURL,
+                             nsIURI*             aBaseURL,
+                             nsIPrincipal*       aSheetPrincipal,
+                             nsCSSValue&         aResult);
+
   // The same as ParseProperty but for a variable.
   void ParseVariable(const nsAString&    aVariableName,
                      const nsAString&    aPropValue,