Rename last two virtual test suites to complete VirtualTestSuite refactoring.

Previously we supported two conventions for naming virtual test suites in
the layout tests. The original convention was to map a subpath onto a
different subpath, e.g. "gpu/compositedscrolling/overflow" would map onto
the tests in "compositing/overflow". Later we started to just use a prefix
plus matching the base subpath exactly, so the prior example would instead be
refered to as "gpu/compositing/overflow".

There were two virtual test suites remaining that used the original syntax.
This patch renames those suites and removes the code that supported the syntax.

gpu/compositedscrolling/overflow -> prefer_composited_to_lcd_text/compositing/overflow
gpu/compositedscrolling/scrollbars -> prefer_composited_to_lcd_text/scrollbars

R=enne@chromium.org, ojan@chromium.org
BUG=410210

Review URL: https://codereview.chromium.org/560563005

git-svn-id: svn://svn.chromium.org/blink/trunk@181695 bbb929c8-8fbe-4397-9dbb-9b2b20218538