browser: Extract ShowLoginWizard() function from browser_dialogs.h

BUG=125846
R=ben@chromium.org

Review URL: https://chromiumcodereview.appspot.com/10377120

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@136811 0039d316-1c4b-4281-b951-d872f2087c98

• chrome/browser/chromeos/chrome_browser_main_chromeos.cc[diff]
• chrome/browser/chromeos/login/base_login_display_host.cc[diff]
• chrome/browser/chromeos/login/login_wizard.h[Added - diff]
• chrome/browser/chromeos/login/wizard_in_process_browser_test.cc[diff]
• chrome/browser/external_tab/external_tab_container_win.cc[diff]
• chrome/browser/ui/views/browser_dialogs.h[diff]
• chrome/chrome_browser.gypi[diff]