Custom titlebar support

[ThaUnknown]

Chrome's window controls overlay defines 4 CSS env variables which are insanely useful for detecting the titlebar size, those are:

  left: env(titlebar-area-x, 0);
  /* Use the environment variable for the top anchoring with a fallback. */
  top: env(titlebar-area-y, 0);
  /* Use the environment variable for setting the width with a fallback. */
  width: env(titlebar-area-width, 100%);
  /* Use the environment variable for setting the height with a fallback. */
  height: env(titlebar-area-height, 33px);

currently gluton doesn't define these, would be cool if it did

[CanadaHonk]

Looks interesting, didn't know that existed for PWAs. Hopefully possible easily via CDP.