Rename PendingSelection to LayoutSelection

This class controls whether LayoutObject.selectionState should be updated.
However its name is ambiguous and we're going to let the class have more
 power to manage LayoutObject. Thus rename it.

BUG=708453

Review-Url: https://codereview.chromium.org/2803583002
Cr-Commit-Position: refs/heads/master@{#462356}
5 files changed