Back button no longer works after site isolation merge

[rgret-dev]

1. Navigate to a site
2. In the same tab, navigate to a separate domain
3. Observe back button is disabled

cc: @trflynn89

[kalenikaliaksandr]

session history data had been moved from browser to webcontent process during adoption of new navigation spec (navigables), because that's was reasonable thing to do at the moment. unfortunately now we see how it breaks if we spawn new webcontent after navigation to a different domain. it will have to be solved by moving session history back to browser process and communicating changes through IPC.