Introduce range-based for loop in LayoutSelection.

LayoutSelection traverses Layout tree in the way here and there.
This CL introduces LayoutSelection::Iterator which lists
 only 'valid' LayoutObjects( see SelectionPaintRange::Iterator::operator++);

BUG=708453

Review-Url: https://codereview.chromium.org/2921863002
Cr-Commit-Position: refs/heads/master@{#478562}
2 files changed