Fix and enable lint checks.

samples/image_diff_png.cc

 // Copyright 2013 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.
 
 // This is a duplicate of chromium's src/tools/imagediff/image_diff_png.cc
 // that has been modified to build in a pdfium environment, which itself
 // was duplicated as follows:
 
 // This is a duplicate of ui/gfx/codec/png_codec.cc, after removing code related
 // to Skia, that we can use when running layout tests with minimal dependencies.
@@ skipping 623 matching lines @@
                    int width,
                    int height,
                    int row_byte_width,
                    bool discard_transparency,
                    std::vector<unsigned char>* output) {
   return Encode(input, FORMAT_BGRA,
       width, height, row_byte_width, discard_transparency,
       std::vector<Comment>(), output);
 }
 
-}  // image_diff_png
+}  // namespace image_diff_png