Move the text-node splitting logic out of Text/CharacterData and into the parser.

This is just another part of:
https://codereview.chromium.org/26129005/
Hopefully with this part landed I'll be able to spot whatever the error
is in the PendingText bits which has prevented me from being able to land
the rest of this CL.

R=abarth@chromium.org
BUG=301981

Review URL: https://codereview.chromium.org/26466007

git-svn-id: svn://svn.chromium.org/blink/trunk@159387 bbb929c8-8fbe-4397-9dbb-9b2b20218538