LayoutTests/fast/canvas/webgl/webgl2-runtime-flag.html

<script>

if (window.testRunner)
    window.testRunner.dumpAsText();

function getContext(name) {
    var c = document.createElement("canvas");
    return c.getContext(name);
}

function run() {
    var output;

    output = document.getElementById("output1");

    if (!window.testRunner) {
        output.textContent = "This test only runs inside DRT/WKTR.";
        return;
    }

    window.internals.settings.setWebGL2Enabled(true);
    var ctx = getContext("webgl2");
    if (ctx)
        output.textContent = "PASS: WebGL 2.0 was available when enabled.";
    else
        output.textContent = "FAIL: WebGL 2.0 was not available when enabled.";

    output = document.getElementById("output2");

    window.internals.settings.setWebGL2Enabled(false);
    var ctx = getContext("webgl2");
    if (ctx)
        output.textContent = "FAIL: WebGL 2.0 was available when disabled.";
    else
        output.textContent = "PASS: WebGL 2.0 was not available when disabled.";
}

window.addEventListener("load", run, false);

</script>
<div id="output1"></div>
<div id="output2"></div>