[LayoutNG] Mark NGBreakToken as const

From a review comment out in crrev.com/c/1258810.

NGBreakToken should be immutable once it was created, but it
was not marked as const.

Bug: 591099
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I16fbe0253ae0d84c7f22be1568a2dac1e6281c25
Reviewed-on: https://chromium-review.googlesource.com/c/1270436
Commit-Queue: Koji Ishii <kojii@chromium.org>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597822}
27 files changed