DevTools: [CSS] remove toggleProperty from protocol.json

This patch removes method toggleProperty from protocol.json. Instead
devtools front-end uses protocol's method setPropertyText.

BUG=120131

Review URL: https://codereview.chromium.org/143473005

git-svn-id: svn://svn.chromium.org/blink/trunk@166530 bbb929c8-8fbe-4397-9dbb-9b2b20218538