Fix a broken test which calls Selection.setPosition(null).

This test was copied from a sibling test:
http://src.chromium.org/viewvc/blink/trunk/LayoutTests/editing/execCommand/indent-images-2.html

This test calls document.getElementsByTagName('div') but there are no div elements.
As a result, test calls Selection.setPosition(null) and it is clearly not intended.
This test uses a span element instead of div element which is targeted in the indent-images-2.html.
Thus this CL rewrite 'div' to 'span'.

This test originally expects that execCommand('indent') should not indent contents inside inline block.
However fixed case, which works as intended, indents the contents.
This CL does only fix broken scripts.

We already started discussion about the execCommand behavior.
If we should change the implementation, we will do it on another patch.

BUG=361911

Review URL: https://codereview.chromium.org/231933004

git-svn-id: svn://svn.chromium.org/blink/trunk@171745 bbb929c8-8fbe-4397-9dbb-9b2b20218538