Introduce SelectionPaintRange in LayoutSelection

SelectionPaintRange has members of |selection_start_|,|selection_end_,
 |selection_start_pos_| and |selection_end_pos_| which LayoutSelection
 had directly.

BUG=708453
TEST=No change in behavior

Review-Url: https://codereview.chromium.org/2901263002
Cr-Commit-Position: refs/heads/master@{#474909}
2 files changed