SimplifiedBackwardsTextIterator shouldn't use bloatedCharacters

Instead, we hold the data in its origin String. This CL makes some of the
clients less efficient because they need to copy the data into a temporary
buffer, but I don't believe the performance of those clients are important.

R=eseidel
BUG=254708

Review URL: https://chromiumcodereview.appspot.com/19772009

git-svn-id: svn://svn.chromium.org/blink/trunk@154543 bbb929c8-8fbe-4397-9dbb-9b2b20218538