selenium-webdriver is not always required for system testing

But `NameError: uninitialized constant ActionDispatch::SystemTesting::Browser::Selenium`
is pretty confused. I've little improved missing constant error to
`NameError: uninitialized constant Selenium`.

actionpack/lib/action_dispatch/system_testing/browser.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require "selenium/webdriver"
-
 module ActionDispatch
   module SystemTesting
     class Browser # :nodoc:
@@ -35,9 +33,9 @@ def capabilities
         @option ||=
           case type
           when :chrome
-            Selenium::WebDriver::Chrome::Options.new
+            ::Selenium::WebDriver::Chrome::Options.new
           when :firefox
-            Selenium::WebDriver::Firefox::Options.new
+            ::Selenium::WebDriver::Firefox::Options.new
           end
       end
 

actionpack/test/dispatch/system_testing/screenshot_helper_test.rb

@@ -3,6 +3,7 @@
 require "abstract_unit"
 require "action_dispatch/system_testing/test_helpers/screenshot_helper"
 require "capybara/dsl"
+require "selenium/webdriver"
 
 class ScreenshotHelperTest < ActiveSupport::TestCase
   test "image path is saved in tmp directory" do