Add a plain space instead of   between text nodes

When we rebalance white spaces,   can be added as space
under some conditions. This CL adds a condition that the next
sibling text node should not exist.

BUG=310149
TEST=editing/inserting/insert-space.html

Review-Url: https://codereview.chromium.org/2175163004
Cr-Commit-Position: refs/heads/master@{#407981}
7 files changed