[9] Review request: 8172988: gradle :web:test should run in headless mode

[Kevin Rushforth]

Hi Dave & Chien,

Please review the following test fix:
https://bugs.openjdk.java.net/browse/JDK-8172988

It fixes a regression in the build script so we will once again run web 
tests in headless mode.

The patch is in the JBS issue.

-- Kevin