This patch allows to insert "%uid" in `IPC_CLIENT_CONNECTION`
declaration and in SystemServer's ini files. This pattern is replaced
then replaced by the UID of the owner of the service. It opens a path
for seamlessly managed, per-user portal.
To achieve this goal:
- The Browser unveils "/tmp/portal/filesystemaccess"
- Pass the page through LoadRequest => ResourceLoader
- ResourceLoader requests a file to the FileSystemAccessServer via IPC
- OutOfProcessWebView handles it and sends a file descriptor back to
the Page.
Also moves WebContentClient and the references to the generated IPC
descriptions, since they are all components of OutOfProcessWebView.
This patch has no functional changes.
