Split LayoutSelecton::ClearSelection() out from SetSelection()

|ClearSelection()| calls |SetSelection(0, -1, 0, -1,
 kPaintInvalidationNewMinusOld)| but what it does is only
 marking old layout object selection state none and invalidating them.
Just extract its step and make |SetSelection()| layout clean.

BUG=708453
TEST=No change in behavior

Review-Url: https://codereview.chromium.org/2894383003
Cr-Commit-Position: refs/heads/master@{#473827}
1 file changed