GPUUploadManagerTest: fix buffer mapping warnings on WebGPU #4157

Job	Run time
Linux -> Pre-checks	19s
Linux -> Clang-NO_GLSLANG-NO_HLSL-x64, Release	5m 27s
Linux -> Clang-NO_GLSLANG-x64, Debug	8m 16s
Linux -> GCC-x64, Release	10m 10s
Linux -> Clang-x64, Release	11m 27s
Linux -> GCC-x64, Debug	12m 9s
Linux -> Clang-x64, Debug	13m 49s
	1h 1m 37s

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