Add virtual/linux-subpixel/ directory with README.txt

This stops lint-webkit-expectations from complaining
that there is no such patch when NoFixTests has a line
containing "virtual/linux-subpixel".

BUG=
TBR=dpranke

Review URL: https://codereview.chromium.org/269713003

git-svn-id: svn://svn.chromium.org/blink/trunk@173162 bbb929c8-8fbe-4397-9dbb-9b2b20218538