Add optional public header checking to GN build

tools/gn/c_include_iterator.h

Index: tools/gn/c_include_iterator.h
diff --git a/tools/gn/c_include_iterator.h b/tools/gn/c_include_iterator.h
new file mode 100644
index 0000000000000000000000000000000000000000..47088fa0793204c6c93a9576fa2bce6344a865ea
--- /dev/null
+++ b/tools/gn/c_include_iterator.h
@@ -0,0 +1,40 @@
+// Copyright 2014 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 TOOLS_GN_C_INCLUDE_ITERATOR_H_
+#define TOOLS_GN_C_INCLUDE_ITERATOR_H_
+
+#include "base/basictypes.h"
+#include "base/strings/string_piece.h"
+
+// Iterates through #includes in C source and header files.
+//
+// This only returns includes we want to check, which is user includes with
+// double-quotes: #include "..."
+class CIncludeIterator {
+ public:
+  // The buffer pointed to must outlive this class.
+  CIncludeIterator(const base::StringPiece& file);
+  ~CIncludeIterator();
+
+  // Fills in the string with the contents of the next include and returns
+  // true, or returns false if there are no more includes.
+  bool GetNextIncludeString(base::StringPiece* out);
+
+ private:
+  // Returns false on EOF, otherwise fills in the given line.
+  bool GetNextLine(base::StringPiece* line);
+
+  base::StringPiece file_;
+
+  size_t offset_;
+
+  // Number of lines we've processed since seeing the last include (or the
+  // beginning of the file) with some exceptions.
+  int lines_since_last_include_;
+
+  DISALLOW_COPY_AND_ASSIGN(CIncludeIterator);
+};
+
+#endif  // TOOLS_GN_INCLUDE_ITERATOR_H_