Make mouse drag to ignore unfocused selection

This patch makes |SelectionController::UpdateSelectionForMouseDrag()| to
ignore unfocused selection to make mouse dragging not to change unfocused
selection.

This patch updates "drag_user_select_none.html" to expect that there are
no selection when starting from "user-select:none".

BUG=481985,714000
TEST=run_layout_test editing/input/drag_in_unselectable.html

Review-Url: https://codereview.chromium.org/2891693002
Cr-Commit-Position: refs/heads/master@{#473130}
3 files changed