Here's what wiki says:
Webgl support on windows
"On Windows with some hardware and driver, WebGL won't work until you copy D3DCompiler_43.dll and d3dx9_43.dll to node-webkit's directory, or install DirectX redistributable.
For license reasons we cannot ship those DLLs."
Some info on those .dll's i found:
Company: Microsoft Corporation
Product Name: Microsoft? DirectX for Windows?
Microsoft? DirectX for Windows?
Direct3D 9 Extensions