[Garden-o-matic] Remove the notion or "platform".

This was left over from pre-Blink days.

R=abarth
BUG=

Review URL: https://codereview.chromium.org/132023011

git-svn-id: svn://svn.chromium.org/blink/trunk@166254 bbb929c8-8fbe-4397-9dbb-9b2b20218538