Add fs_config entries for the webview_zygote.

The webview_zygote is a non-root zygote process that creates isolated_app
children for rendering web content. It needs:
- CAP_SETUID and CAP_SETGID to change the UID of the new child process.
- CAP_SETPCAP to clear the capability bounding set after forking.

Test: m
Test: angler boots

Bug: 21643067
Change-Id: I986fa04be54e812f5dd2afa14e5d2d3e474e2b10