Use SelectionPaintRange::Iterator in LayoutSelection::ClearSelection().

ClearSelection() only uses LayoutObjects that just SelectionPaintRange
 iterates.

BUG=708453
TEST=No change in behavior

Review-Url: https://codereview.chromium.org/2936963003
Cr-Commit-Position: refs/heads/master@{#479304}
1 file changed