Convert editing/selection/5234383-[12].html to use w3c test harness

This patch converts "editing/selection/5234383-[12].html" to use w3c
test harness to simplify test files and consolidate into
"move_by_sentence_boundary.html" with additional test cases to improve test
coverage.

This patch is a preparation of getting rid of grammar checking feature.

BUG=619452
TEST=n/a; this patch changes a test file

Review-Url: https://codereview.chromium.org/2184473006
Cr-Commit-Position: refs/heads/master@{#408057}
15 files changed