Updated fragment shader sample to use uniform-by-name accessor

[gaaclarke]

Do not land until the following are on stable:

• impeller: allows access of float uniforms by name flutter#176728
• Implements uniform-by-name for web flutter#176980

Pre-launch Checklist

• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I read the Contributors Guide.
• I have added sample code updates to the changelog.
• I updated/added relevant documentation (doc comments with ///).

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[gaaclarke]

I think these things are making it to stable. Unfortunately it looks like there is a bug in vulkan implementation though (flutter/flutter#180595). So it's probably not worth sending people to this new API quite yet.