Allow content layer to pass ProtocolInterceptors when we create URLRequestContext

Currently we pass protocol_handlers that belong to content layer
in StoragePartitionImplMap when we create URLRequestContexts for
storage partitions.

This patch also allows content layer to pass additional protocol
interceptors, so that content modules / web platform modules
(namely, ServiceWorker) can insert its own protocol interceptors.

Most changes other than in chrome/browser/profiles/profile_io_data.cc
are just for plumbing.

* The additional protocol interceptors are created in:
 content/browser/storage_partition_impl_map.cc
 (where we also create protocol_handlers for content modules,
 e.g. blob, filesystem etc)

* which is passed to: ChromeContentBrowserContext::CreateRequestContext
 -> Profile -> ProfileIOData, and then

* chained to the URLJobFactory for a newly created
 URLRequestContext in ProfileIOData::SetUpJobFactoryDefaults().

BUG=349319