[browserstack benchmark tool] batch benchmark code snippet

[Linchenn]

This PR enables users add code snippets to benchmark into preconfigured_browser.json. Then the app.js program will iterate each code snippet to benchmark.

An example usage:

1. update the "benchmark" value in preconfigured_browser.json with the following:

{
    "model": ["mobilenet_v2", "mesh_128"],
    "numRuns": 10,
    "backend": ["webgl"],
    "codeSnippets": [{"codeSnippet":"predict = () => { return tf.conv2d(img, filter, 1, 'same');};",
                      "setupCodeSnippetEnv":"img = tf.randomUniform([1, 240, 240, 3], 0, 1000); filter = tf.randomUniform([3, 3, 3, 3], 0, 1000);"}]
  }

2. run node app.js --benchmark='./preconfigured_browser.json' --cloud --outfile=js and you could see benchmark results for the code snippet in benchmark_results.html

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This change is 

[lina128]

LGTM

Reviewable status: complete! 1 of 1 approvals obtained (waiting on @pyu10055)

[pyu10055]

Reviewed 3 of 3 files at r1.
Reviewable status: complete! 2 of 1 approvals obtained