Mark fast/forms/placeholder-position.html as flaky

Looks like this has been flaky for a long time.
The failure is in the exact pixel drawing of transform text in a form field.

BUG=304952
R=enne@chromium.org

Review URL: https://codereview.chromium.org/26360002

git-svn-id: svn://svn.chromium.org/blink/trunk@159047 bbb929c8-8fbe-4397-9dbb-9b2b20218538