Add raw pixel serialization fallback for SkImages that cannot be encoded.

This fixes serialize-8888 for 2 GMs on Mac that I'm now unblacklisting.
I think another was already fixed, and two more were Windows-only.

Seems safe to use encoded_size=1 as another sentinel here (like we already
use =0); I can't imagine any encoded image format that can encode an image
in a single byte.

I suspect this is the root of the referenced bug too,
but this is a good idea even if not.

BUG=chromium:601851
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2039813007

CQ_EXTRA_TRYBOTS=client.skia:Test-Win-MSVC-GCE-CPU-AVX2-x86_64-Release-Trybot,Test-Mac-Clang-MacMini6.2-CPU-AVX-x86_64-Release-Trybot;client.skia.android:Test-Android-GCC-Nexus5-CPU-NEON-Arm7-Release-Trybot,Test-Android-GCC-Nexus9-CPU-Denver-Arm64-Release-Trybot

Review-Url: https://codereview.chromium.org/2039813007
4 files changed