Add --force-compositing-mode to some virtual test suites.

This adds it to the following suites:
virtual/gpu/fast/canvas
virtual/softwarecompositing
virtual/gpu/compositedscrolling/overflow
virtual/gpu/compositedscrolling/scrollbars

R=enne, ojan
BUG=361729, 371886

Review URL: https://codereview.chromium.org/276003002

git-svn-id: svn://svn.chromium.org/blink/trunk@173773 bbb929c8-8fbe-4397-9dbb-9b2b20218538