Added layout test for printing of WebGL-rendered canvases.

Depends on: https://codereview.chromium.org/348413002

This test would have caught a regression in the printing of Google Maps
filed under Issue 351956. Verified by backporting this test and the
associated infrastructure to an older workspace, ensuring the test
failed, and that the test passed with (one of) the associated bug fixes.

In order to catch the regression it must be run with the threaded
compositor enabled. Added virtual/threaded/printing to the set of
virtual test suites and added suppressions for two tests which fail in
that mode.

BUG=387298

Review URL: https://codereview.chromium.org/339433005

git-svn-id: svn://svn.chromium.org/blink/trunk@176885 bbb929c8-8fbe-4397-9dbb-9b2b20218538