Avoid duplicate functions/code in core/editing: kUnsetCodePoint

While experimenting with unity builds I encountered a few duplicate
symbols and functions in core/editing. This patch renames, moves
and unifies them.

kInvalidCodePoint is used in both ForwardGraphemeBoundaryStateMachine and
BackwardGraphemeBoundaryStateMachine so to avoid clashes, use different
names for them.

BUG=708949
R=yosin@chromium.org

Review-Url: https://codereview.chromium.org/2801893004
Cr-Commit-Position: refs/heads/master@{#462423}
1 file changed