TextIterator: Adding more unit tests for <input>.

This patch adds unit tests to TextIteratorTest which test TextIterator's
behavior on <input> elements.

This patch is in preparation for a later patch which will change how <input>
is treated in TextIterator.

Only adding new unit tests; no change in production code.

BUG=244955

Review URL: https://codereview.chromium.org/103743002

git-svn-id: svn://svn.chromium.org/blink/trunk@163204 bbb929c8-8fbe-4397-9dbb-9b2b20218538