Unity build preprocessing script to enable Disk Size with LTO code optimization for WebGL builds.

PreprocessBuild.cs

using UnityEditor.Build;
using UnityEditor.Build.Reporting;
using UnityEngine;

public class PreprocessBuild : IPreprocessBuildWithReport
{
    public int callbackOrder { get; }

    public void OnPreprocessBuild(BuildReport report)
    {
        Debug.Log("Preprocessing build.");

#if UNITY_WEBGL
        // We only want to change user build settings for automated builds, not for local development.
        // If we didn't check for batch mode, this would override the developer's preferred code optimization setting
        // (which is probably "Shorter Build Time").
        if (Application.isBatchMode)
        {
            Debug.Log("Setting WebGL code optimization to Disk Size with LTO.");
            UnityEditor.WebGL.UserBuildSettings.codeOptimization = UnityEditor.WebGL.WasmCodeOptimization.DiskSizeLTO;
        }
#endif
    }
}