Download the host components when user tries to enable Me2Me host.

Now when NPAPI plugin is not present the host setup wizard detects
when the host is not installed and asks the user to install it.

BUG=339626

Review URL: https://codereview.chromium.org/132793007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248604 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248604