Turn down warning level for GN build of image_diff utility

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 201 matching lines @@
         state->output_channels = 3;
         break;
       case FORMAT_RGBA:
         state->row_converter = &ConvertRGBtoRGBA;
         state->output_channels = 4;
         break;
       case FORMAT_BGRA:
         state->row_converter = &ConvertRGBtoBGRA;
         state->output_channels = 4;
         break;
+      default:
+        NOTREACHED();
+        break;
     }
   } else if (channels == 4) {
     switch (state->output_format) {
       case FORMAT_RGB:
         state->row_converter = &ConvertRGBAtoRGB;
         state->output_channels = 3;
         break;
       case FORMAT_RGBA:
         state->row_converter = NULL;  // no conversion necessary
         state->output_channels = 4;
         break;
       case FORMAT_BGRA:
         state->row_converter = &ConvertBetweenBGRAandRGBA;
         state->output_channels = 4;
         break;
+      default:
+        NOTREACHED();
+        break;
     }
   } else {
     NOTREACHED();
     longjmp(png_jmpbuf(png_ptr), 1);
   }
 
   state->output->resize(
       state->width * state->output_channels * state->height);
 }
 
@@ skipping 308 matching lines @@
       if (discard_transparency) {
         output_color_components = 3;
         png_output_color_type = PNG_COLOR_TYPE_RGB;
         converter = ConvertBGRAtoRGB;
       } else {
         output_color_components = 4;
         png_output_color_type = PNG_COLOR_TYPE_RGB_ALPHA;
         converter = ConvertBetweenBGRAandRGBA;
       }
       break;
+
+    default:
+      NOTREACHED();
+      return false;
   }
 
   // Row stride should be at least as long as the length of the data.
   if (input_color_components * width < row_byte_width)
     return false;
 
   png_struct* png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING,
                                                 NULL, NULL, NULL);
   if (!png_ptr)
     return false;
@@ skipping 50 matching lines @@
                    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