Fix code that relied on readPixels not doing color space conversion

SampleApp doesn't have (can't easily get) an image, so I couldn't use
the new helper function there. It's probably still worth having?

BUG=skia:

Change-Id: I60c208ff958076015a9539359921b9aff68f25c8
Reviewed-on: https://skia-review.googlesource.com/7129
Reviewed-by: Matt Sarett <msarett@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
4 files changed