Refactor LayoutSelection::SelectionBounds() with CollectSelectedMap().

CollectSelectedMap() collects LayoutObjects and LayoutBlocks that removed for-while-loop iterates.
Then we can unify their Rect.

BUG=708453
TEST=No change in behavior

Review-Url: https://codereview.chromium.org/2938133002
Cr-Commit-Position: refs/heads/master@{#479643}
1 file changed