Revert 139933 - Clean up RenderViewHostManager swapping logic.
Reverting due to http://crbug.com/131376.

Makes the difference between swapping SiteInstances and swapping
BrowsingInstances explicit, and adds CHECKs to enforce invariants
more effectively.

BUG=123007
TEST=No functionality change.

Review URL: https://chromiumcodereview.appspot.com/9965091

TBR=creis@chromium.org
Review URL: https://chromiumcodereview.appspot.com/10546029

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@140795 0039d316-1c4b-4281-b951-d872f2087c98
8 files changed