Add command-line arguments to js2gtest generated tests.
Add command-line arguments to js2gtest generated tests.

This will allow testing behavior behind a flag using
generated WebUIBrowserTests (specifically, this is to
unblock Polymer browser tests for projects behind flags,
e.g. --enable-md-settings or --enable-media-router).

js2gtest tests can specify their flags on their test
fixture's prototype, e.g.:
  ...
  browsePreload: 'chrome://md-settings',
  commandLineSwitches: [{
    switchName: '--some-switch',
    switchValue: 'someValue'
  }, {
    switchName: '--enable-boolean-switch'
  }],
  ...

Review URL: https://codereview.chromium.org/1196513002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#335894}

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