Browsers start with Chromium already running

A headless browser at the default window size (1280 by 800) is now created from a snapshot of Chromium that is already running, instead of starting Chromium in the new sandbox, which took 1 to 3 seconds. Each browser gets its own CDP secret, its own random state, and an empty profile. A server builds the snapshot the first time a browser of that size is created on it, so that first browser starts as before. Other settings start Chromium as before.