Make SkPngCodec support rewinding properly.

Make SkPngCodec support rewinding properly.

Separate out the code for reading the header, and use it to reinitialize
fPng_ptr and fInfo_ptr after a rewind.

Use common code to clean up fPng_ptr and fInfo_ptr, and set them to
NULL and treat them as NULL as appropriate.

Update the test to expect SkPngCodec to succeed.

BUG=skia:3257
Committed: https://skia.googlesource.com/skia/+/3eada2a49ff2814833afcbd61329aa451d86da9c

[scroggo, 2015-04-01 13:01:51]

scroggo@google.com changed reviewers:
+ halcanary@google.com, msarett@google.com

[msarett, 2015-04-01 15:58:37]

lgtm

I don't want to hold this up any longer, but it might be good to get a look from
Hal as well.

[scroggo, 2015-04-01 16:28:08]

The CQ bit was checked by scroggo@google.com

[commit-bot: I haz the power, 2015-04-01 16:28:22]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1048423003/1

[commit-bot: I haz the power, 2015-04-01 16:33:29]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/3eada2a49ff2814833afcbd61329aa451d86da9c