Revert "Allow conversion from non-opaque to opaque"

This reverts commit bfe8dca7dfdd9cafbccba0a637f2fcd58c7a54fb.

Reason for revert: I believe I've fixed all the call sites,
so this is now ok.

Original change's description:
> Allow conversion from non-opaque to opaque
> 
> BUG:683787
> 
> Change-Id: I1b78cc8d1b5d3917a2a952da036b93022e99e053
> Reviewed-on: https://skia-review.googlesource.com/7374
> Reviewed-by: Robert Phillips <robertphillips@google.com>
> Commit-Queue: Matt Sarett <msarett@google.com>
> 

TBR=msarett@google.com,robertphillips@google.com,reviews@skia.org
# Not skipping CQ checks because original CL landed > 1 day ago.

Change-Id: I2369c9b81321ca83a7063bb85e66ddbc03914d2b
Reviewed-on: https://skia-review.googlesource.com/8073
Commit-Queue: Matt Sarett <msarett@google.com>
Reviewed-by: Matt Sarett <msarett@google.com>
1 file changed