Fix FIXME in findPreviousAndNextAttributes in RenderSVGText.cpp

Use the existing pre-order traversal methods on RenderObject to implement
the traversal in an iterative fashion instead of as a recursion.

This allows moving the |stopAfterNext| boolean to the scope of
findPreviousAndNextAttributes, so do that and fix up the callers.

BUG=

Review URL: https://codereview.chromium.org/141703017

git-svn-id: svn://svn.chromium.org/blink/trunk@165950 bbb929c8-8fbe-4397-9dbb-9b2b20218538