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
Access editingViewPort shadow element with ID

This change reduces source code size and memory consumption.

BUG=none
TEST=none; no behavior changes.
R=ksakamoto@chromium.org

Review URL: https://codereview.chromium.org/23708054

git-svn-id: svn://svn.chromium.org/blink/trunk@158078 bbb929c8-8fbe-4397-9dbb-9b2b20218538