Clean up RenderViewHostManager swapping logic.

Makes the difference between swapping SiteInstances and swapping
BrowsingInstances explicit.

BUG=123007
TEST=No functionality change.

Review URL: https://codereview.chromium.org/76063007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237388 0039d316-1c4b-4281-b951-d872f2087c98
7 files changed