Remove some dumb Vector copies in RenderGrid

Allowing implicit copying of Vector is just evil and should
be stopped! These didn't show up on any profiles and were
found by code inspection.

BUG=273238

Review URL: https://chromiumcodereview.appspot.com/24267010

git-svn-id: svn://svn.chromium.org/blink/trunk@158077 bbb929c8-8fbe-4397-9dbb-9b2b20218538