Add commandline flag for service workers.

this allows the browser to detect at runtime if service workers should be enabled or not.

BUG=285976

Review URL: https://codereview.chromium.org/27238005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228721 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228721