For non-e10s Service Worker, we use Cache entries to achieve interception.
While this is a temporary measure, the fact that cache enforces size limits on
cache entries (which make sense for the purpose it was designed) prevents large
files from being served via a Service Worker. This patch adds a skip size check
flag to CacheStorage that is relayed to CacheEntry and CacheFile. It is set to
false by default leading to normal cache behaviour.
The patch also adds nsICacheStorageService.synthesizedCacheStorage() that
retrieves a cache storage with this flag set to true, which is used by
nsHttpChannel in case of possible interception.
