Add new hostingType for startProcessLocked() that selects using the WebViewZygote.

Test: m
Test: angler boots
Test: Turn on Multiprocess WebView in Developer Options and launch the WebView
      Shell. The sandboxed_service parent process is the webview_zygote.

Bug: 21643067
Change-Id: I9dab548853372fd91f6bbe204cc8686ef2e94448