Remove deprecated "decoration" getter and setter accessors from WebTextCheckingResult.

Chromium is not using them, nor Blink, so they can be just removed now.

BUG=None
TEST=None, no functional changes.
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/27014002

git-svn-id: svn://svn.chromium.org/blink/trunk@159523 bbb929c8-8fbe-4397-9dbb-9b2b20218538