This patch implements the File API spec's supplemental steps for
document's "unloading document cleanup steps" so that we now remove blob
URLs associated with the document's relevant settings object when the
document is being unloaded.
Fixes two realm leaks when running our test suite.
The Blob URL store is intended to be a singleton across all WebContent
instances. But for now, this implements a per-WebContent store, which
only lives as long as the WebContent process itself.