Convert editing/selection/5057506.html to use w3c test harness

This patch converts "editing/selection/5057506.html" to use w3c
test harness to simplify test script, and rename
"double_click_after_last_cell.html" to describe what this test script does.

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/2178193004
Cr-Commit-Position: refs/heads/master@{#408037}
8 files changed