This patch adds a new IPDL protocol PBackgroundLocalStorageCache. It is used by LocalStorageCache object to broadcast changes in local storage cache to other content processes. Each origin has its own PBackgroundLocalStorageCache, so now we can notify content processes that actually have a local storage cache for given origin. This greatly improves performance and reduces memory footprint especialy when local storage changes carry big strings and/or happen very quickly (before this patch all child processes were blindly notified).
- stop inheriting StorageDBBridge in StorageDBThread and StorageDBChild
- move StorageDBThread and StorageDBChild initialization out of LocalStorageCache
- use IPC even for the intra-process communication in main process
- rationalize a bit storage observer code
- make StorageDBParent to always be created and destroyed on the background thread
