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Rather than route this through RemoteWindowTreeHostWin (which can now
be NULL) I'm routing it through ChromeMetroViewerProcessHost. I'm also
adding a NULL check as it may be possible in some code paths for the
host to have been disconnected and therefor NULL.

BUG=374766
TEST=none
R=ananta@chromium.org

Review URL: https://codereview.chromium.org/297713002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@271816 0039d316-1c4b-4281-b951-d872f2087c98