Implement correct logic for URLPatternSet set operators.

extensions/common/url_pattern_set.h

Index: extensions/common/url_pattern_set.h
diff --git a/extensions/common/url_pattern_set.h b/extensions/common/url_pattern_set.h
index da6ad132db183433d726363f5d291b9e412e0158..4ee7b42c3efdd0e3ee0edbafe6f01d5945ce7e8b 100644
--- a/extensions/common/url_pattern_set.h
+++ b/extensions/common/url_pattern_set.h
@@ -59,8 +59,9 @@ class URLPatternSet {
   const_iterator begin() const { return patterns_.begin(); }
   const_iterator end() const { return patterns_.end(); }
 
-  // Adds a pattern to the set. Returns true if a new pattern was inserted,
-  // false if the pattern was already in the set.
+  // Adds a pattern to the set. If a more general pattern is added, the more
+  // specific ones are removed from the set. Returns true if a new pattern was
+  // inserted, false if the pattern was already in the set.
   bool AddPattern(const URLPattern& pattern);
 
   // Adds all patterns from |set| into this.
@@ -96,6 +97,10 @@ class URLPatternSet {
                 std::string* error);
 
  private:
+  // Directly inserts a pattern in the set without checking for more
+  // generic/more specific containment conditions.
+  bool InsertPattern(const URLPattern& pattern);
+
   // The list of URL patterns that comprise the extent.
   std::set<URLPattern> patterns_;
 };