Add the "webview_zygote" domain.

The webview_zygote is a new unprivileged zygote and has its own sockets for
listening to fork requests. However the webview_zygote does not run as root
(though it does require certain capabilities) and only allows dyntransition to
the isolated_app domain.

Test: m
Test: angler boots

Bug: 21643067
Change-Id: I89a72ffe6dcb983c4a44048518efd7efb7ed8e83