With WKBasedNavigationManager enabled, evicting a background tab's WKWebView cause it to lose its history (including its current URL), so switching back to that tab just loads 'about:blank'.

To reproduce this, I'm force-evicting the background tab's WKWebView by "kill -9"-ing its WebContent process -- I'm not sure if there's a nicer way to do that (though tests do this by toggling TabModel.webUsageEnabled).

This is also causing TabUsageRecorderTestCase tests to fail (even after fixing the races covered by  issue 775645 ), since they call EvictOtherTabModelTabs and expect to still be able to switch back to those tabs (but now when they switch back they're unexpectedly winding up on about:blank).

Do we need to do something like make a copy of the WKBackForwardList before the tab goes into the background?