Improve accessible text retrieved from generated content.

For a Text node, AXRenderObject::stringValue returns the plainText
of the node's text, using TextIterator.

Now it does the same thing even when a RenderText doesn't have an
associated Text node, by iterating over the inline text boxes and
then converting all whitespace to plain whitespace characters.
(Previously it just returned the raw text, including extra whitespace,
newlines, etc.)

In the future, a better solution would be to have TextIterator handle
generated content. That would be a huge change. This is a decent
alternative in the meantime.

BUG=98977

Review URL: https://codereview.chromium.org/36123002

git-svn-id: svn://svn.chromium.org/blink/trunk@160943 bbb929c8-8fbe-4397-9dbb-9b2b20218538