Add Shadow DOM-related tests to TextIteratorTest.

This patch adds a few unit tests to TextIteratorTest, each of which tests
TextIterator's behavior on shadow trees.

This patch does not touch TextIterator's implementation. This change just adds
tests to capture the current behavior.

BUG=244955

Review URL: https://codereview.chromium.org/79083004

git-svn-id: svn://svn.chromium.org/blink/trunk@162599 bbb929c8-8fbe-4397-9dbb-9b2b20218538