Disallow an empty host in a CSP host-source directive

Currently "https://" is accepted and treated like "https:". This behavior has never been part of any standard.

The syntax is specified in http://www.w3.org/TR/CSP11/#source-list-syntax

host-source       = [ scheme-part "://" ] host-part [ port-part ] [ path-part ]
host-part         = "*" / [ "*." ] 1*host-char *( "." 1*host-char )

As you can see, the host-part is NOT optional.

BUG=404295

Review URL: https://codereview.chromium.org/470723006

git-svn-id: svn://svn.chromium.org/blink/trunk@180407 bbb929c8-8fbe-4397-9dbb-9b2b20218538