Disambiguate extension dialog string.

Add a meaning tag to IDS_EXTENSION_PROMPT_WARNING_HOSTS_MANY to disambiguate
it from IDS_EXTENSION_PROMPT_WARNING_HOSTS_DEFAULT, which has the same text
but is used in a different plural case.

BUG=342370
TBR=joi@chromium.org

Review URL: https://codereview.chromium.org/192773005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255961 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255961