Cleanup: Access 'container' shadow element in text fields with ID.

This change reduces source code size and memory consumption.

BUG=
TEST=none; no behavior changes.

Review URL: https://chromiumcodereview.appspot.com/24210016

git-svn-id: svn://svn.chromium.org/blink/trunk@158247 bbb929c8-8fbe-4397-9dbb-9b2b20218538