Show unescaped CSS strings when hovering over them

Demo: https://mathiasbynens.github.io/css-dbg-stories/css-escapes.html

Bug: chromium:1033910
Change-Id: Ic9495bc93b9e95139905db57894af4b3403c73a8
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/1975889
Reviewed-by: Changhao Han <[email protected]>
Commit-Queue: Mathias Bynens <[email protected]>
diff --git a/front_end/externs.js b/front_end/externs.js
index e6ec1ca..51aba0f 100644
--- a/front_end/externs.js
+++ b/front_end/externs.js
@@ -987,6 +987,12 @@
 const base64ToSize = function(content) {};
 
 /**
+ * @param {?string} input
+ * @return {string}
+ */
+const unescapeCssString = function(input) {};
+
+/**
  * @constructor
  * @param {function(!Array<*>)} callback
  */