codegen: use device descriptors

[arjunattam]

npx playwright-cli --device="Pixel 2" codegen wikipedia.org

Currently, this is generated as

(async() => {
  const browser = await chromium.launch({
    headless: false
  });
  const context = await browser.newContext({
    userAgent: 'Mozilla/5.0 (Linux; Android 8.0; Pixel 2 Build/OPD3.170816.012) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/75.0.3765.0 Mobile Safari/537.36',
    viewport: {
      width: 411,
      height: 731
    },
    isMobile: true,
    hasTouch: true,
    defaultBrowserType: 'chromium'
  });

It would be great to see this as

(async() => {
  const browser = await chromium.launch({
    headless: false
  });
  const context = await browser.newContext({
   ...devices['Pixel 2']
  });