Add the webview zygote socket to the zygote whitelist.

Allow the webview zygote socket to be open at time of fork, so that
webview renderers can be forked successfully.

(cherry picked from commit a7fcb2bc)

Test: WebView functions correctly with multiprocess dev setting enabled
Bug: 21643067
Change-Id: I312846433a43a8e4548cb2f98b96b56acf9f5333