Use --disable-gpu for virtual/softwarecompositing test suite.

This will let us remove the --enable-software-compositing and
the --disable-gpu-compositing command line flags from chromium.

R=jamesr
BUG=350141

Review URL: https://codereview.chromium.org/189503002

git-svn-id: svn://svn.chromium.org/blink/trunk@168972 bbb929c8-8fbe-4397-9dbb-9b2b20218538