tf.enableDebugMode() cause an error

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TensorFlow.js version: 1.7.2
tfjs-wechat plugin version: 0.0.9
WeChat version: 微信开发者工具
WeChat base API version: 2.10.4
WeChat IDE version: 1.02.2004020
Describe the problem or feature request: calling of tfjs.enableDebugMode makes the tfjs access BOM object, which are not accessible in wechat. that behaviour causes Cannot read property 'userAgent' of undefined

Code to reproduce the bug / link to feature request

the code:

    tf.enableDebugMode()

the error

Uncaught (in promise) thirdScriptError
Cannot read property 'userAgent' of undefined
TypeError: Cannot read property 'userAgent' of undefined
    at Object.evaluationFn (http://127.0.0.1:26467/appservice/miniprogram_npm/@tensorflow/tfjs-core/index.js:17:73072)
    at t.evaluateFlag (http://127.0.0.1:26467/appservice/miniprogram_npm/@tensorflow/tfjs-core/index.js:17:3160)
    at t.get (http://127.0.0.1:26467/appservice/miniprogram_npm/@tensorflow/tfjs-core/index.js:17:2468)
    at t.getNumber (http://127.0.0.1:26467/appservice/miniprogram_npm/@tensorflow/tfjs-core/index.js:17:2546)
    at e.startTimer (http://127.0.0.1:26467/appservice/miniprogram_npm/@tensorflow/tfjs-core/index.js:17:294533)
    at e.runWebGLProgram (http://127.0.0.1:26467/appservice/miniprogram_npm/@tensorflow/tfjs-core/index.js:17:334813)
    at e.uploadToGPU (http://127.0.0.1:26467/appservice/miniprogram_npm/@tensorflow/tfjs-core/index.js:17:337786)
    at http://127.0.0.1:26467/appservice/miniprogram_npm/@tensorflow/tfjs-core/index.js:17:333339
    at Array.map (<anonymous>)
    at e.runWebGLProgram (http://127.0.0.1:26467/appservice/miniprogram_npm/@tensorflow/tfjs-core/index.js:17:332630)

what causes the error(i'm using minified code of tfjs)

navigator.userAgent || navigator.vendor || window.opera