Certificate Transparency: Enhance the CT log flag for multiple logs

Allow specifying multiple CT logs through the command line. This is
necessary in order to allow testing of multiple new logs at the same
time.

BUG=

Review URL: https://codereview.chromium.org/148283002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247685 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247685