web-view: wait 10 seconds before killing unresponsive process

WebKit determines that the web process is hung when it has been more than three seconds since it has responded to a ping from the UI process. Turns out, this timeout is too low in practice because websites routinely run JavaScript for longer than this. So let's wait 10 seconds after the process is determined to be unresponsive, for 13 seconds total, before we kill anything. My hope is that this makes the unresponsive process killer less painful in practice.

We might still want to ask users before killing anything, #1561 (closed), but this might suffice for now. Maybe.