When decorating the preedit string, stop forcing the cursor past the end of the input

Change-Id: Id71b0ef81692437a4d5e65b548aecebd0578da38

BUG=chromium-os:8758
TEST=Set the IME to chewing.  click on the omnibox.  type 'ch' then hit space.  Ensure that garbled characters don't appear.

Review URL: http://codereview.chromium.org/6312141
1 file changed