Remove the implicit fallback to DIRECT when proxies fail. This better matches other browsers, and simplifies the code.

      To better understand what this means, here are some examples how the behaviors will differ for the user:

(1) You start chrome with --proxy-server=&quotfoobar:80&quot.
The server &quotfoobar:80&quot is refusing connections.

  Before: Would fallback to direct after failing to connect through foobar:80.
  Now: Will error-out with connection refused after failing to connect through foobar:80.

(2) You start chrome with --proxy-pac-url=&quotfile:///foobar.pac&quot.
The server &quotfoobar:80&quot is unreachable, and foobar.pac reads:
  function FindProxyForURL(url, host) {
    return &quotPROXY foobar:80"
  }

  Before: Would fallback to direct after failing to connect through foobar:80.
  Now: Will error-out with connection refused after failing to connect through foobar:80.

(3) You start chrome with --proxy-pac-url=&quotfile:///foobar.pac&quot.
The server &quotfoobar:80&quot is unreachable, and foobar.pac reads:
  function FindProxyForURL(url, host) {
    return &quotPROXY foobar:80; DIRECT"
  }

  *No change, since the fallback to DIRECT is explicit in the PAC script*

BUG=12303

Review URL: http://codereview.chromium.org/502068

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@35549 0039d316-1c4b-4281-b951-d872f2087c98