Failed to run on Android Chrome

[hideya]

Execution fails with the following error at the start up (during the warm-up) and when clicking "Guess" button.

A TF.js demo, Emoji Scavenger Hunt, that uses similar features, runs fine on Android Chrome, So, there should be a way to fix it.

Android version tested is 6.0 and TF.js is of version_core: "0.9.1". I don't have newer Android device, but I'm informed that Android 8.0 device failed to run this demo in the same manner.

Uncaught Error: Failed to compile fragment shader.
    at Object.createFragmentShader (webpack:///./node_modules/@tensorflow/tfjs-core/dist/kernels/webgl/webgl_util.js?:91)
    at GPGPUContext.createProgram (webpack:///./node_modules/@tensorflow/tfjs-core/dist/kernels/webgl/gpgpu_context.js?:166)
    at Object.compileProgram (webpack:///./node_modules/@tensorflow/tfjs-core/dist/kernels/webgl/gpgpu_math.js?:25)
    at eval (webpack:///./node_modules/@tensorflow/tfjs-core/dist/kernels/backend_webgl.js?:775)
    at MathBackendWebGL.getAndSaveBinary (webpack:///./node_modules/@tensorflow/tfjs-core/dist/kernels/backend_webgl.js?:791)
    at MathBackendWebGL.compileAndRun (webpack:///./node_modules/@tensorflow/tfjs-core/dist/kernels/backend_webgl.js?:774)
    at MathBackendWebGL.conv2d (webpack:///./node_modules/@tensorflow/tfjs-core/dist/kernels/backend_webgl.js?:690)
    at environment_1.ENV.engine.runKernel.x (webpack:///./node_modules/@tensorflow/tfjs-core/dist/ops/conv.js?:83)
    at Engine.runKernel (webpack:///./node_modules/@tensorflow/tfjs-core/dist/engine.js?:72)
    at ConvOps.conv2d (webpack:///./node_modules/@tensorflow/tfjs-core/dist/ops/conv.js?:83)

[bfbca]

Perhaps GPU runs out memory. This happens on older Android smartphone like Galaxy S5, but works on newer Galaxy S7 and Pixel 2 XL.

Adding tf.setBackend('cpu') as 1st line in window.onload in index.js fixes the crash on Galaxy S5, however recognition gets slow, around 10sec.

[hideya]

Thanks for the input and the fix! Yeah, it looks that Issue #2 is also caused by out-of-GPU-memory.
Merged your PR. Thank you!