Force scalefactor=1 for gentest to fix issues with test generation on hdpi devices

Summary:
Since my new main work maschine is a hdpi device, all the gentests with rounding are producing different outputs.

This PR force the scalefactor=1 for gentest to fix issues with generation on hdpi devices.
Closes https://github.com/facebook/yoga/pull/434

Differential Revision: D4604724

Pulled By: emilsjolander

fbshipit-source-id: ce4036ae71a45775280e0504f20c06a4622ccceb

gentest/gentest.rb

@@ -9,7 +9,7 @@ caps = Selenium::WebDriver::Remote::Capabilities.chrome(
     "performance"=>"ALL"
   }
 )
-browser = Watir::Browser.new(:chrome, :desired_capabilities => caps)
+browser = Watir::Browser.new(:chrome, :desired_capabilities => caps, :switches => ['--force-device-scale-factor=1', '--window-position=0,0'])
 Dir.chdir(File.dirname($0))
 
 Dir['fixtures/*.html'].each do |file|