[LayoutNG] Add BoxType to distinguish different box fragments

Bug: 591099
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: I2f6d01a6351b07d8bc93ad016858f49f549cf89c
Reviewed-on: https://chromium-review.googlesource.com/722383
Commit-Queue: Koji Ishii <kojii@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509687}
2 files changed