Disable proxy discovery in headless mode.

[alancnet]

Corrects an issue where HeadlessChrome downloads scripts at a very slow rate. As demonstrated in the screenshot, when running headless, each script took several seconds to load, causing minutes of delay for a test run. This may not occur in all environments. Shown is Chromium 70.0.3508.0 on Windows 7 SP1.

Note: This does not occur if the --headless flag is removed, so this may be indicative of a bug in Chromium.

[alancnet]

CI failure unrelated to PR:

The command "eval npm install  " failed. Retrying, 3 of 3.
/home/travis/.nvm/versions/node/v4.9.1/lib/node_modules/npm/bin/npm-cli.js:85
      let notifier = require('update-notifier')({pkg})
      ^^^
SyntaxError: Block-scoped declarations (let, const, function, class)
             not yet supported outside strict mode

[rogeriopvl]

@alancnet thanks for submitting this PR and sorry for the delay responding.

It's been a while since this PR was created. Can you confirm the issue still occurs?

[alancnet]

@rogeriopvl Sorry for the delay. After Thanksgiving, I've been occupied in another project. I will revisit this as soon as I can.