Don't use --enable-software-compositing flag in virtual test.

It's now enabled by default on all platforms and unnecessary.

BUG=350141

Review URL: https://codereview.chromium.org/235243005

git-svn-id: svn://svn.chromium.org/blink/trunk@171708 bbb929c8-8fbe-4397-9dbb-9b2b20218538