LibWeb: Implement an ephemeral Blob URL store

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.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -206,6 +206,7 @@ set(SOURCES
     Fetch/Request.cpp
     Fetch/Response.cpp
     FileAPI/Blob.cpp
+    FileAPI/BlobURLStore.cpp
     FileAPI/File.cpp
     FileAPI/FileList.cpp
     FontCache.cpp