Remove redundant variables in LayoutSelection::SetSelection()

Use |selection_{start/end}_pos_| instead of old_{start/end}_pos|
 and update |selection_{start/end}_pos_| at the end of SetSelection().

TEST=No change in behavior
BUG=708453

Review-Url: https://codereview.chromium.org/2884903002
Cr-Commit-Position: refs/heads/master@{#472365}
1 file changed