Introduce CollectSelectedMap() in LayoutSelection::SetSelection()

This CL introduces local static CollectSelectedMap() from big LayoutSelection::SetSelection()
 and shares it to create |{new,old}_selected_map|.

TEST=No change in behavior.
BUG=708453

Review-Url: https://codereview.chromium.org/2874153002
Cr-Commit-Position: refs/heads/master@{#471247}
1 file changed