Rename LayoutTests/platform/chromium-linux* to LayoutTests/platform/linux*

BUG=231188
R=ojan@chromium.org

Review URL: https://codereview.chromium.org/18182002

git-svn-id: svn://svn.chromium.org/blink/trunk@153308 bbb929c8-8fbe-4397-9dbb-9b2b20218538