Roll engine 39a950e5d1d1..480844608c16 (2 commits) (#32016)

https://github.com/flutter/engine/compare/39a950e5d1d1...480844608c16

git log 39a950e5d1d1cbeb3d70334c6275d224e843b30e..480844608c16287c968198c394c5adac6def983c --no-merges --oneline
480844608 Remove asserts and add BuildConfig (flutter/engine#8821)
c97bd4543 Download the Fuchsia SDK and toolchain in a gclient hook. (flutter/engine#8794)

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bin/internal/engine.version

-39a950e5d1d1cbeb3d70334c6275d224e843b30e
+480844608c16287c968198c394c5adac6def983c