The length of scheme is at least five characters even when the scheme has 'web+' prefix

The content-scheme handler specification requires that it is SecurityError if the length of
scheme isn't five characters at least, including 'web+' prefix. Thus, we need to check the length
of scheme on the 'web+' scheme.

Spec: http://www.whatwg.org/specs/web-apps/current-work/#custom-handlers

BUG= N/A

Review URL: https://chromiumcodereview.appspot.com/22985005

git-svn-id: svn://svn.chromium.org/blink/trunk@156049 bbb929c8-8fbe-4397-9dbb-9b2b20218538