Ensure error page SiteInstance is not reused on successful navigations.

This CL fixes a problem with the logic whether the current SiteInstance
should be reused on navigation. It was accounting for error page
isolation only in one direction - when going from regular page to an
error page. The fix ensures that when the navigation is successful
and the current SiteInstance is for the error page, a SiteInstance
check is performed as well.

Bug: 866549
Change-Id: Ie8e297ca578130fbbfc7bbb952da407222113055
Reviewed-on: https://chromium-review.googlesource.com/c/1274899
Commit-Queue: Nasko Oskov <[email protected]>
Reviewed-by: Charlie Reis <[email protected]>
Cr-Commit-Position: refs/heads/master@{#598973}
2 files changed