Access placeholder shadow element with ID.

It's safer than holding pointers in the host element. We reduce code size and
memory consumption.

BUG=none
TEST=none; no behavior changes.

Review URL: https://chromiumcodereview.appspot.com/23570012

git-svn-id: svn://svn.chromium.org/blink/trunk@158005 bbb929c8-8fbe-4397-9dbb-9b2b20218538