Auto merge of #185 - servo:…, r=emilio

Fix interaction of skip_whitespace and nested blocks

This fixes (at least some of) the test failures at servo/servo#18171

In code like this:

```css
unsupported selector ! { stuff }
valid selector { color: green }
```

`rules_and_declarations::parse_qualified_rule` would leave the parser just after the first `{` with `Parser::at_start_of == Some(BlockType::CurlyBracket)`. The latter means that, unless `Parser::parse_nested_block` is called, the parser needs to skip over this block until the matching `}` before doing anything else. This PR makes `Parser::skip_whitespace` and `Parser::skip_cdc_and_cdo` (both added recently in #181) take care of `Parser::at_start_of` correctly the same way `Parser::next` does.

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Author: bors-servo

src/parser.rs

@@ -300,17 +300,19 @@ impl<'i: 't, 't> Parser<'i, 't> {
     /// Advance the input until the next token that’s not whitespace or a comment.
     #[inline]
     pub fn skip_whitespace(&mut self) {
-        // If we just consumed `{`, `[`, `(`, or `function(`, leave whitespace
-        // or comments inside the block or function up to the nested parser.
-        if self.at_start_of.is_some() {
-            return
+        if let Some(block_type) = self.at_start_of.take() {
+            consume_until_end_of_block(block_type, &mut self.input.tokenizer);
         }
 
         self.input.tokenizer.skip_whitespace()
     }
 
     #[inline]
     pub(crate) fn skip_cdc_and_cdo(&mut self) {
+        if let Some(block_type) = self.at_start_of.take() {
+            consume_until_end_of_block(block_type, &mut self.input.tokenizer);
+        }
+
         self.input.tokenizer.skip_cdc_and_cdo()
     }