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