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..0fec249432d8d89fb953734033c601759458c1b7 100644
--- a/ui/gfx/codec/png_codec.cc
+++ b/ui/gfx/codec/png_codec.cc
@@ -712,8 +712,11 @@ bool EncodeWithCompressionLevel(const unsigned char* input,
                                                 NULL, NULL, NULL);
   if (!png_ptr)
     return false;
+  // info_ptr should have wider lifetime scope than destroyer that access it,
+  // to avoid use-after-scope bugs.
+  png_info* info_ptr;
   PngWriteStructDestroyer destroyer(&png_ptr);

[dcheng, 2016/12/14 22:32:47]

Can we move this line down to after return false? Then we can combine
declaration + initialization of |info_ptr| (and we may as well just make
&info_ptr an argument to the ctor).

[krasin1, 2016/12/14 23:45:04]

Daniel, Nico,

I have made a simplification that can be applied to other types of Png*Destroyer
classes too (and I am fine to do the upgrade in this CL, it you feel it's worth
it).

Basically, instead of making this delicate dance inline, I have added two fields
into PngWriteStructDestroyer (and also renamed it to PngWriteStructInfo, as it
now holds png_ptr and info_ptr), and we use the fields instead of local
variables.

Do you feel that's less fragile?

-  png_info* info_ptr = png_create_info_struct(png_ptr);
+  info_ptr = png_create_info_struct(png_ptr);
   if (!info_ptr)
     return false;
   destroyer.SetInfoStruct(&info_ptr);