Five playground tests crash with SIGABRT when impeller_unittests runs on a Windows host:

• Play/AiksTest.CanRenderEmojiTextFrame/Vulkan
• Play/AiksTest.CanRenderEmojiTextFrameWithBlur/Vulkan
• Play/AiksTest.CanRenderEmojiTextFrameWithAlpha/Vulkan
• Play/TypographerTest.LazyAtlasTracksColor/Vulkan
• Play/TypographerTest.GlyphColorIsPartOfCacheKey/Vulkan

The crash is a Skia hard assert:

SkScalerContext_win_dw.cpp(1724): fatal error:
"assertf(SkCodecs::HasDecoder("png")): No PNG decoder registered.
A call to SkCodecs::Register is necessary."

These tests rasterize color emoji with the host's font stack. On Windows that is Skia's DirectWrite backend, which decodes PNG-format color glyphs (for example from Segoe UI Emoji) through the SkCodecs registry. The engine runtime registers the codecs in ImageGeneratorRegistry, but test binaries must register them for themselves; impeller_unittests never does, so the first color glyph aborts the process. Linux CI takes the FreeType scaler path and never triggers the decode, which is why this only surfaces on Windows hosts.

The fix registers the PNG decoder once in PlaygroundTest::SetUp(), matching the local-registration pattern already used by lib/ui/painting/image_decoder_no_gl_unittests.cc. All five tests pass on a Windows host with this change; behavior on other hosts is unchanged (the registration is additive and idempotent for the running binary).

Fixes #189565

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