Fix a performance issue in nextBoundary and previousBoundary

When iterating through DOM nodes nextBoundary and previousBoundary
convert the contents of nodes using text security to a sequence of 'x'
characters. The SimplifiedBackwardsTextIterator and TextIterator may
iterate past node boundaries. Before this patch, the transformation was
done looking at the starting node rather than the current node. In some
situations, this replaced all boundaries with 'x' and caused the
iterator to continue iterating and transforming until the extent of the
document.

BUG=121918

Review URL: https://codereview.chromium.org/26626003

git-svn-id: svn://svn.chromium.org/blink/trunk@159358 bbb929c8-8fbe-4397-9dbb-9b2b20218538