Tag images as sRGB by default in SkCodec

Tag images as sRGB by default in SkCodec

Unmarked images should be treated as sRGB.

BUG=skia:4895

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4151

Change-Id: I5f8c308d22fd2d069cbfa89c5a5bb19ae6fde8bd
Reviewed-on: https://skia-review.googlesource.com/4151
Reviewed-by: Leon Scroggins <scroggo@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Matt Sarett <msarett@google.com>