[rb] Add helper method to launch Chrome in headless mode.

This makes it consistent with Firefox::Options#headless!

Signed-off-by: Alex Rodionov <p0deje@gmail.com>

rb/lib/selenium/webdriver/chrome/options.rb

@@ -89,6 +89,8 @@ def add_encoded_extension(encoded)
         #
 
         def add_argument(arg)
+          return if @args.include?(arg)
+
           @args << arg
         end
 
@@ -122,6 +124,18 @@ def add_preference(name, value)
           prefs[name] = value
         end
 
+        #
+        # Run Chrome in headless mode.
+        #
+        # @example Enable headless mode
+        #   options = Selenium::WebDriver::Chrome::Options.new
+        #   options.headless!
+        #
+
+        def headless!
+          add_argument '--headless'
+        end
+
         #
         # Add an emulation device name
         #

rb/spec/integration/selenium/webdriver/chrome/options_spec.rb

@@ -47,6 +47,26 @@ module Chrome
             expect(ua).to eq('foo;bar')
           end
         end
+
+        context 'headless mode' do
+          it 'should be able to run in headless mode with #headless!' do
+            subject.headless!
+
+            create_driver!(options: subject) do |driver|
+              ua = driver.execute_script 'return window.navigator.userAgent'
+              expect(ua).to match(/HeadlessChrome/)
+            end
+          end
+
+          it 'should be able to run in headless mode with #add_argument' do
+            subject.add_argument('--headless')
+
+            create_driver!(options: subject) do |driver|
+              ua = driver.execute_script 'return window.navigator.userAgent'
+              expect(ua).to match(/HeadlessChrome/)
+            end
+          end
+        end
       end
     end # Chrome
   end # WebDriver

rb/spec/unit/selenium/webdriver/chrome/options_spec.rb

@@ -93,6 +93,35 @@ module Chrome
             subject.add_argument('foo')
             expect(subject.args).to include('foo')
           end
+
+          context 'multiple arguments' do
+            it 'should add only unique arguments' do
+              subject.add_argument('foo')
+              subject.add_argument('bar')
+
+              expect(subject.args).to eq %w[foo bar]
+            end
+
+            it 'should not add same argument more than once' do
+              subject.add_argument('foo')
+              subject.add_argument('foo')
+
+              expect(subject.args).to eq ['foo']
+            end
+          end
+        end
+
+        describe '#headless!' do
+          it 'should add the --headless command-line argument' do
+            subject.headless!
+            expect(subject.args).to eql ['--headless']
+          end
+
+          it 'should not add the --headless command-line argument if already present' do
+            subject.add_argument('--headless')
+            subject.headless!
+            expect(subject.args).to eql ['--headless']
+          end
         end
 
         describe '#add_option' do