Add security comment for the InjectedScriptHost.

BUG=242322
R=yurys@chromium.org

Review URL: https://codereview.chromium.org/15808005

git-svn-id: svn://svn.chromium.org/blink/trunk@151122 bbb929c8-8fbe-4397-9dbb-9b2b20218538