Use -1 for initial value for text offset instead of 0 for nextBoundary() in VisualUnit.cpp

This patch changes initial value of text offset for |next| variable of nextBoundary() in VisualUnit.cpp for checking whether |next| is set value or not. Variable |next| is set as value of searchFunction().

BUG=271235
TEST=LayoutTests/editing/inserting/insert-character-in-overflow-crash.html
R=tkent@chromium.org

Review URL: https://codereview.chromium.org/25228004

git-svn-id: svn://svn.chromium.org/blink/trunk@158575 bbb929c8-8fbe-4397-9dbb-9b2b20218538