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Currently we crash if one renderer has a plugin and another one has a plugin that gets destroyed, and started again (e.g. reload). We get away with reload on a single page currently because the process gets torn down.
This properly cleans up the socket and channel mapping on both sides, so it can be restarted.

Review URL: http://codereview.chromium.org/155174


git-svn-id: svn://svn.chromium.org/chrome/trunk/src@20112 0039d316-1c4b-4281-b951-d872f2087c98