Disallow readPixels() conversions from untagged srcs to tagged dsts

This might break the roll into Chrome.  But let's see how.

BUG=skia:6021

Change-Id: I2698b5d6fe72d01bed0dc64703b592a03d441a80
Reviewed-on: https://skia-review.googlesource.com/7127
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Matt Sarett <msarett@google.com>
2 files changed