Implement FilePath::Contains()

base/file_path.h

Index: base/file_path.h
diff --git a/base/file_path.h b/base/file_path.h
index 9049176265f53ddee786c9c54fafe58400859223..cfe0f2ac095d93b84343218e4a167ecb6f14ce9c 100644
--- a/base/file_path.h
+++ b/base/file_path.h
@@ -159,6 +159,11 @@ class FilePath {
   // platforms, an absolute path begins with a separator character.
   bool IsAbsolute() const;
 
+  // Returns true if this FilePath represents a parent dir of |other|. Both
+  // paths are normalized before doing the comparison, but neither |this| nor
+  // |other| are modified.
+  bool Contains(const FilePath& other) const;
+
   // Returns a copy of this FilePath that does not end with a trailing
   // separator.
   FilePath StripTrailingSeparators() const;