Switch SkCodec to int for counts and indices

Switch SkCodec to int for counts and indices

This matches other Skia APIs. size_t was adopted from blink/
GIFImageReader.

Change-Id: Ic83e59f0942f597c4fb834e623acd9886ad483fe
Reviewed-on: https://skia-review.googlesource.com/13274
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Matt Sarett <msarett@google.com>
Reviewed-by: Chris Blume <cblume@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>