DevTools: [CSS] use setStyleText as undo action for setPropertyText

This patch changes SetPropertyTextAction to use setStyleText method
instead of setPropertyText for the undo operation. This
makes it possible to successfully undo multiple property insertions.

BUG=341506

Review URL: https://codereview.chromium.org/211193006

git-svn-id: svn://svn.chromium.org/blink/trunk@170206 bbb929c8-8fbe-4397-9dbb-9b2b20218538