Finish removing bloatedCharacters

This removes the last vestiges of bloatedCharacters, including the API for
creating them. A future CL will remove the underlying mechanism for storing the
16 bit shadow buffer.

R=eseidel
BUG=254708

Review URL: https://chromiumcodereview.appspot.com/19729012

git-svn-id: svn://svn.chromium.org/blink/trunk@154559 bbb929c8-8fbe-4397-9dbb-9b2b20218538