Make SkImage_Base::onReadPixels() pure virtual

BUG=skia:

Change-Id: I52504b21313717bf8321ec3f1df770773703a1a1
Reviewed-on: https://skia-review.googlesource.com/6831
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Matt Sarett <msarett@google.com>
2 files changed