Revert "Replace surrogates in JS strings with U+FFFD instead of panicking."

This reverts commit 3f07f8e866.

components/script/dom/bindings/conversions.rs

@@ -355,9 +355,7 @@ pub fn jsstring_to_str(cx: *mut JSContext, s: *mut JSString) -> DOMString {
         let char_vec = unsafe {
             slice::from_raw_parts(chars as *const u16, length as usize)
         };
-        // This is a wilful spec violation.
-        // See https://github.com/servo/servo/issues/6564
-        String::from_utf16_lossy(char_vec)
+        String::from_utf16(char_vec).unwrap()
     }
 }