Rework MediaType to be an atom-based struct instead of an enum.

MozReview-Commit-ID: 1Tfrs9PBkhA

components/style/str.rs

@@ -8,6 +8,7 @@
 
 use num_traits::ToPrimitive;
 use std::ascii::AsciiExt;
+use std::borrow::Cow;
 use std::convert::AsRef;
 use std::iter::{Filter, Peekable};
 use std::str::Split;
@@ -151,3 +152,13 @@ pub fn starts_with_ignore_ascii_case(string: &str, prefix: &str) -> bool {
     string.len() >= prefix.len() &&
       string.as_bytes()[0..prefix.len()].eq_ignore_ascii_case(prefix.as_bytes())
 }
+
+/// Returns an ascii lowercase version of a string, only allocating if needed.
+pub fn string_as_ascii_lowercase<'a>(input: &'a str) -> Cow<'a, str> {
+    if input.bytes().any(|c| matches!(c, b'A'...b'Z')) {
+        input.to_ascii_lowercase().into()
+    } else {
+        // Already ascii lowercase.
+        Cow::Borrowed(input)
+    }
+}