Remove innerSpinButtonElement() and speechButtonElement() from HTMLInputElement.

Use ShadowRoot::getElementById instead.

BUG=

Review URL: https://chromiumcodereview.appspot.com/22879005

git-svn-id: svn://svn.chromium.org/blink/trunk@156088 bbb929c8-8fbe-4397-9dbb-9b2b20218538