Use Parser::skip_whitespace in a few places to make Parser::try rewind less.

Gecko’s CSS parsing microbenchmarks before:

```
  43.437 ±  0.391 ms    Stylo.Servo_StyleSheet_FromUTF8Bytes_Bench
  29.244 ±  0.042 ms    Stylo.Gecko_nsCSSParser_ParseSheet_Bench
 281.884 ±  0.028 ms    Stylo.Servo_DeclarationBlock_SetPropertyById_Bench
 426.242 ±  0.008 ms    Stylo.Servo_DeclarationBlock_SetPropertyById_WithInitialSpace_Bench
```

After:

```
  29.779 ±  0.254 ms    Stylo.Servo_StyleSheet_FromUTF8Bytes_Bench
  28.841 ±  0.031 ms    Stylo.Gecko_nsCSSParser_ParseSheet_Bench
 296.240 ±  4.744 ms    Stylo.Servo_DeclarationBlock_SetPropertyById_Bench
 293.855 ±  4.304 ms    Stylo.Servo_DeclarationBlock_SetPropertyById_WithInitialSpace_Bench
```

components/style/Cargo.toml

@@ -37,7 +37,7 @@ bitflags = "0.7"
 bit-vec = "0.4.3"
 byteorder = "1.0"
 cfg-if = "0.1.0"
-cssparser = "0.19"
+cssparser = "0.19.3"
 encoding = {version = "0.2", optional = true}
 euclid = "0.15"
 fnv = "1.0"

components/style/properties/properties.mako.rs

@@ -1547,8 +1547,12 @@ impl PropertyDeclaration {
                               id: PropertyId, context: &ParserContext, input: &mut Parser<'i, 't>)
                               -> Result<(), PropertyDeclarationParseError<'i>> {
         assert!(declarations.is_empty());
+        let start = input.state();
         match id {
             PropertyId::Custom(name) => {
+                // FIXME: fully implement https://github.com/w3c/csswg-drafts/issues/774
+                // before adding skip_whitespace here.
+                // This probably affects some test results.
                 let value = match input.try(|i| CSSWideKeyword::parse(i)) {
                     Ok(keyword) => DeclaredValueOwned::CSSWideKeyword(keyword),
                     Err(()) => match ::custom_properties::SpecifiedValue::parse(context, input) {
@@ -1561,11 +1565,11 @@ impl PropertyDeclaration {
                 Ok(())
             }
             PropertyId::Longhand(id) => {
+                input.skip_whitespace();  // Unnecessary for correctness, but may help try() rewind less.
                 input.try(|i| CSSWideKeyword::parse(i)).map(|keyword| {
                     PropertyDeclaration::CSSWideKeyword(id, keyword)
                 }).or_else(|()| {
                     input.look_for_var_functions();
-                    let start = input.state();
                     input.parse_entirely(|input| id.parse_value(context, input))
                     .or_else(|err| {
                         while let Ok(_) = input.next() {}  // Look for var() after the error.
@@ -1592,6 +1596,7 @@ impl PropertyDeclaration {
                 })
             }
             PropertyId::Shorthand(id) => {
+                input.skip_whitespace();  // Unnecessary for correctness, but may help try() rewind less.
                 if let Ok(keyword) = input.try(|i| CSSWideKeyword::parse(i)) {
                     if id == ShorthandId::All {
                         declarations.all_shorthand = AllShorthand::CSSWideKeyword(keyword)
@@ -1603,7 +1608,6 @@ impl PropertyDeclaration {
                     Ok(())
                 } else {
                     input.look_for_var_functions();
-                    let start = input.state();
                     // Not using parse_entirely here: each ${shorthand.ident}::parse_into function
                     // needs to do so *before* pushing to `declarations`.
                     id.parse_into(declarations, context, input).or_else(|err| {