Fix stack-use-after-scope in png_codec.

ui/gfx/codec/png_codec.cc

Index: ui/gfx/codec/png_codec.cc
diff --git a/ui/gfx/codec/png_codec.cc b/ui/gfx/codec/png_codec.cc
index ac386ad91d0b656e146776c8077694c881719e99..d6f49d1934f2ad916d27100d1c4d1ee9b3299203 100644
--- a/ui/gfx/codec/png_codec.cc
+++ b/ui/gfx/codec/png_codec.cc
@@ -328,22 +328,20 @@ class PngReadStructDestroyer {
   DISALLOW_COPY_AND_ASSIGN(PngReadStructDestroyer);
 };
 
-// Automatically destroys the given write structs on destruction to make
-// cleanup and error handling code cleaner.
-class PngWriteStructDestroyer {
+// Holds png struct and info ensuring the proper destruction.
+class PngWriteStructInfo {
  public:
-  explicit PngWriteStructDestroyer(png_struct** ps) : ps_(ps), pi_(0) {
-  }
-  ~PngWriteStructDestroyer() {
-    png_destroy_write_struct(ps_, pi_);
+  explicit PngWriteStructInfo() : png_str_(0), info_ptr_(0) {

[Nico, 2016/12/14 23:47:52]

(minor nit: "str" means "string" for me, not struct, so I actually like ps_ a
bit more - png_struct_ is probably even better)

[dcheng, 2016/12/14 23:48:52]

Minor nit: no explicit, and use nullptr

[krasin1, 2016/12/14 23:53:47]

Done.

   }
-  void SetInfoStruct(png_info** pi) {
-    pi_ = pi;
+
+  ~PngWriteStructInfo() {
+    png_destroy_write_struct(&png_str_, &info_ptr_);
   }
+
+  png_struct* png_str_;
+  png_info* info_ptr_;
  private:
-  png_struct** ps_;
-  png_info** pi_;
-  DISALLOW_COPY_AND_ASSIGN(PngWriteStructDestroyer);
+  DISALLOW_COPY_AND_ASSIGN(PngWriteStructInfo);
 };
 
 bool BuildPNGStruct(const unsigned char* input, size_t input_size,
@@ -708,15 +706,15 @@ bool EncodeWithCompressionLevel(const unsigned char* input,
   // Row stride should be at least as long as the length of the data.
   DCHECK(input_color_components * size.width() <= row_byte_width);
 
-  png_struct* png_ptr = png_create_write_struct(PNG_LIBPNG_VER_STRING,
-                                                NULL, NULL, NULL);
-  if (!png_ptr)
+  PngWriteStructInfo si;
+  si.png_str_ = png_create_write_struct(PNG_LIBPNG_VER_STRING,
+                                        NULL, NULL, NULL);
+  if (!si.png_str_)
     return false;
-  PngWriteStructDestroyer destroyer(&png_ptr);
-  png_info* info_ptr = png_create_info_struct(png_ptr);
-  if (!info_ptr)
+
+  si.info_ptr_ = png_create_info_struct(si.png_str_);
+  if (!si.info_ptr_)
     return false;
-  destroyer.SetInfoStruct(&info_ptr);
 
   output->clear();