Revert "Revert "Disallow readPixels() conversions from untagged srcs to tagged dsts""

Revert "Revert "Disallow readPixels() conversions from untagged srcs to tagged dsts""

This reverts commit 8461506c8b025522eb89cd754c63fb5dbf0cb7f1.

Reason for revert: Fixed some Chrome code, let's try this again.

Original change's description:
> Revert "Disallow readPixels() conversions from untagged srcs to tagged dsts"
>
> This reverts commit ccfd1083a7b9d546735ddd1037f3240635cce72d.
>
> Reason for revert: Roll is failing.
>
> Original change's description:
> > 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>
> >
>
> TBR=msarett@google.com,brianosman@google.com,reed@google.com
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=skia:6021
>
> Change-Id: I4b62178fd7c23f43cf69ca69fc14526ecd503956
> Reviewed-on: https://skia-review.googlesource.com/7205
> Commit-Queue: Matt Sarett <msarett@google.com>
> Reviewed-by: Matt Sarett <msarett@google.com>
>

TBR=msarett@google.com,brianosman@google.com,reed@google.com
# Not skipping CQ checks because original CL landed > 1 day ago.
BUG=skia:6021

Change-Id: I3f3f33fe6b84fbd5c537b60ed5c8b9201d529a6a
Reviewed-on: https://skia-review.googlesource.com/8156
Commit-Queue: Matt Sarett <msarett@google.com>
Reviewed-by: Matt Sarett <msarett@google.com>