Take 2 at implementing activeTab.

chrome/common/extensions/url_pattern_set.h

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_COMMON_EXTENSIONS_URL_PATTERN_SET_H_
 #define CHROME_COMMON_EXTENSIONS_URL_PATTERN_SET_H_
 #pragma once
 
 #include <set>
 
@@ skipping 31 matching lines @@
 
   URLPatternSet();
   URLPatternSet(const URLPatternSet& rhs);
   explicit URLPatternSet(const std::set<URLPattern>& patterns);
   ~URLPatternSet();
 
   URLPatternSet& operator=(const URLPatternSet& rhs);
   bool operator==(const URLPatternSet& rhs) const;
 
   bool is_empty() const;
+  size_t size() const;
   const std::set<URLPattern>& patterns() const { return patterns_; }
   const_iterator begin() const { return patterns_.begin(); }
   const_iterator end() const { return patterns_.end(); }
 
   void AddPattern(const URLPattern& pattern);
   void ClearPatterns();
 
   // Returns true if the permission |set| is a subset of this.
   bool Contains(const URLPatternSet& set) const;
 
@@ skipping 11 matching lines @@
                 int valid_schemes,
                 bool allow_file_access,
                 std::string* error);
 
  private:
   // The list of URL patterns that comprise the extent.
   std::set<URLPattern> patterns_;
 };
 
 #endif  // CHROME_COMMON_EXTENSIONS_URL_PATTERN_SET_H_