Revert of Remove SK_SUPPORT_LEGACY_DEEPFLATTENING. (patchset #1 id:1 of https://coder...

Revert of Remove SK_SUPPORT_LEGACY_DEEPFLATTENING. (patchset #1 id:1 of https://codereview.chromium.org/769953002/)

Reason for revert:
Breaks canary builds.  Will reland after the Chromium change lands.

Original issue's description:
> Remove SK_SUPPORT_LEGACY_DEEPFLATTENING.
>
> This was needed for pictures before v33, and we're now requiring v35+.
>
> Will follow up with the same for skia/ext/pixel_ref_utils_unittest.cc
>
> BUG=skia:
>
> Committed: https://skia.googlesource.com/skia/+/52c293547b973f7fb5de3c83f5062b07d759ab88

TBR=reed@google.com,mtklein@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Review URL: https://codereview.chromium.org/768183002