Use RAII for write libpng structs.

ui/gfx/codec/png_codec.cc

Index: ui/gfx/codec/png_codec.cc
===================================================================
--- ui/gfx/codec/png_codec.cc	(revision 130345)
+++ ui/gfx/codec/png_codec.cc	(working copy)
@@ -359,8 +359,27 @@
  private:
   png_struct** ps_;
   png_info** pi_;
+  DISALLOW_COPY_AND_ASSIGN(PngReadStructDestroyer);
 };
 
+// Automatically destroys the given write structs on destruction to make
+// cleanup and error handling code cleaner.
+class PngWriteStructDestroyer {
+ public:
+  explicit PngWriteStructDestroyer(png_struct** ps) : ps_(ps), pi_(0) {
+  }
+  ~PngWriteStructDestroyer() {
+    png_destroy_write_struct(ps_, pi_);
+  }
+  void SetInfoStruct(png_info** pi) {
+    pi_ = pi;
+  }
+ private:
+  png_struct** ps_;
+  png_info** pi_;
+  DISALLOW_COPY_AND_ASSIGN(PngWriteStructDestroyer);
+};
+
 bool BuildPNGStruct(const unsigned char* input, size_t input_size,
                     png_struct** png_ptr, png_info** info_ptr) {
   if (input_size < 8)
@@ -746,18 +765,17 @@
                                                 NULL, NULL, NULL);
   if (!png_ptr)
     return false;
+  PngWriteStructDestroyer destroyer(&png_ptr);
   png_info* info_ptr = png_create_info_struct(png_ptr);
-  if (!info_ptr) {
-    png_destroy_write_struct(&png_ptr, NULL);
+  if (!info_ptr)
     return false;
-  }
+  destroyer.SetInfoStruct(&info_ptr);
 
   PngEncoderState state(output);
   bool success = DoLibpngWrite(png_ptr, info_ptr, &state,
                                size.width(), size.height(), row_byte_width,
                                input, compression_level, png_output_color_type,
                                output_color_components, converter, comments);
-  png_destroy_write_struct(&png_ptr, &info_ptr);
 
   return success;
 }