Convert testRunner.display to displayAsync(Then)

This removes all callers of the synchronous display() call so that it
can be removed on the Chromium side.

See also: https://codereview.chromium.org/224743002

BUG=345027

Review URL: https://codereview.chromium.org/224093012

git-svn-id: svn://svn.chromium.org/blink/trunk@171158 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove platform overrides virtual_test_suites

Change the virtual linux-subpixel from being only being defined on
linux to being a platform-independent suite skipped on all other
platforms as our tooling isn't set up to handle platform specific
virtual test suites.

This removes the only platform override of virtual_test_suites thus
having all virtual test suites defined in base.py

R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/231233003

git-svn-id: svn://svn.chromium.org/blink/trunk@171159 bbb929c8-8fbe-4397-9dbb-9b2b20218538