Part 1. Duplicate IsLocalRefURL to nsContentUtils to reuse this function.

`IsLocalRefURL` is originally designed to be used by URLValue only.
Since we need this function in SVGUseElement::LookupHref too, duplicate it to nsContentUtils as a util function.
This is a duplication because CSSValue uses stringbuffers and not nsStrings.
While Bug 1356060 - "Just use nsString in URLValueData" converts this use from stringbuffer to nsString, it builds on a bunch of vartype refactoring (nsString vs. nsAString, etc.) which is too much of a headache to deal with just to deduplicate this simple function.

dom/base/nsContentUtils.cpp

@@ -9787,3 +9787,19 @@ nsContentUtils::AttemptLargeAllocationLoad(nsIHttpChannel* aChannel)
 
   return reloadSucceeded;
 }
+
+/* static */ bool
+nsContentUtils::IsLocalRefURL(const nsString& aString)
+{
+  // Find the first non-"C0 controls + space" character.
+  const char16_t* current = aString.get();
+  for (; *current != '\0'; current++) {
+    if (*current > 0x20) {
+      // if the first non-"C0 controls + space" character is '#', this is a
+      // local-ref URL.
+      return *current == '#';
+    }
+  }
+
+  return false;
+}

dom/base/nsContentUtils.h

@@ -2730,6 +2730,13 @@ public:
 
   static bool AttemptLargeAllocationLoad(nsIHttpChannel* aChannel);
 
+  /**
+   * Detect whether a string is a (CSS) local-url.
+   * https://drafts.csswg.org/css-values/#local-urls
+   */
+  static bool
+  IsLocalRefURL(const nsString& aString);
+
 private:
   static bool InitializeEventTable();