Implement surrounding text API for exo::TextInput

Bug: 826614
Test: exo_unittests
Change-Id: Ia2fe9a39c38b0ea93d1f0a0f5aa820536a656a08
Reviewed-on: https://chromium-review.googlesource.com/c/1399722
Commit-Queue: Jun Mukai <mukai@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Yuichiro Hanada <yhanada@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629807}
4 files changed