[browserstack benchmark tool] Move CodeSnippet from server to client

[Linchenn]

This PR moves CodeSnippet to benchmark from server sider to client side.

Specifically: Previously, CodeSnippet is hard-coded in a server file (benchmark_model.js), which will be executed by BrowserStack instances. This PR moves CodeSnippet to client side, and, let the client side to pass the CodeSnippet to the server side.

After this PR, we could add UI elements to enable users to edit codeSnippet (client side).

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