Remove deprecated enum type from WebHTTPBody.h

The corresponding Chromium change have been already landed, so it should be fine
to remove this now.

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

Review URL: https://chromiumcodereview.appspot.com/23983024

git-svn-id: svn://svn.chromium.org/blink/trunk@157563 bbb929c8-8fbe-4397-9dbb-9b2b20218538