[LayoutNG] Introduce CanUseInlineBox() for tracking project progress

This patch introduces |CanUseInlineBox()| as a preparation to add
|DCHECK(CanUseInlineBox())| to functions using |InlineBox| for marker of
progress of LayoutNG project. To release LayoutNG, we should not hit
these |DCHECK()|.

Following patch will utilize |CanUseInlineBox()| into layout/ and editing/.

The patch[1] is a sample of utilization.

[1] http://crrev.com/c/763188 [LayoutNG] Utilize DCHECK(CanUseInlineBox()) in
core/layout/


Bug: 591099, 707656
Change-Id: Id71c39ae8520ab18fd1b8b8f3e18a66e8a7d567c
Reviewed-on: https://chromium-review.googlesource.com/768572
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516239}
2 files changed