Turn down warning level for GN build of image_diff utility

samples/image_diff_png.cc

Index: samples/image_diff_png.cc
diff --git a/samples/image_diff_png.cc b/samples/image_diff_png.cc
index a54dc816cfc82cb86fc3e6a929dd60012c9a4215..04c2591b608bfbb3f5718214b9240a91c87a1514 100644
--- a/samples/image_diff_png.cc
+++ b/samples/image_diff_png.cc
@@ -219,6 +219,9 @@ void DecodeInfoCallback(png_struct* png_ptr, png_info* info_ptr) {
         state->row_converter = &ConvertRGBtoBGRA;
         state->output_channels = 4;
         break;
+      default:
+        NOTREACHED();
+        break;
     }
   } else if (channels == 4) {
     switch (state->output_format) {
@@ -234,6 +237,9 @@ void DecodeInfoCallback(png_struct* png_ptr, png_info* info_ptr) {
         state->row_converter = &ConvertBetweenBGRAandRGBA;
         state->output_channels = 4;
         break;
+      default:
+        NOTREACHED();
+        break;
     }
   } else {
     NOTREACHED();
@@ -562,6 +568,10 @@ bool EncodeWithCompressionLevel(const unsigned char* input, ColorFormat format,
         converter = ConvertBetweenBGRAandRGBA;
       }
       break;
+
+    default:
+      NOTREACHED();
+      return false;
   }
 
   // Row stride should be at least as long as the length of the data.