Synchronously handle only background cached responses (experimental)

[nicksay]

To avoid the forced 1s delay in WebKit browsers when handling cached responses when in the background, synchronously execute the handler function instead of using a setTimeout of 0, but only if document.webkitVisibilityState is 'hidden'. For evaluation and testing, guard this behavior behind a experimental-sync-response-cache-background config flag.

Progress on #337

[nicksay]

New commit description:

To avoid the forced 1s delay in WebKit browsers when handling cached responses when in the background, synchronously execute the handler function instead of using a setTimeout of 0, but only if document.hidden is true. Note that this still will apply the synchronous behavior to all browsers, but limit the behavior to tabs that aren't visible. For evaluation and testing, guard this behavior behind a experimental-sync-response-cache-background config flag.

[rviscomi]

LGTM