Separate renderer-specific code in InputType::attach.

Introduce new function InputType::countUsage, and move UseCounter code to
it. InputType::attach implementations have only renderer-specific code.

This is a preparation to support author Shadow DOM for INPUT element.

BUG=274205
TEST=none; no behavior changes

Review URL: https://chromiumcodereview.appspot.com/23330003

git-svn-id: svn://svn.chromium.org/blink/trunk@156380 bbb929c8-8fbe-4397-9dbb-9b2b20218538