Update gen-filenames.js to generate posix paths (#18488)

script/gen-filenames.js

@@ -68,7 +68,7 @@ const main = async () => {
       // Remove whitespace
       .map(line => line.trim())
       // Get the relative path
-      .map(line => path.relative(rootPath, line))
+      .map(line => path.relative(rootPath, line).replace(/\\/g, '/'))
       // Only care about files in //electron
       .filter(line => !line.startsWith('..'))
       // Only care about our own files