Use new downcast helper for blink::LayoutText and its subclasses

This CL converts DEFINE_LAYOUT_OBJECT_TYPE_CASTS() to DowncastTraits<>
for:
- LayoutBR
- LayoutCounter
- LayoutText
- LayoutTextCombine
- LayoutTextFragment
- LayoutNGText

This CL has no behavior changes.

Bug: 891908
Change-Id: I495349f1efffa3f9a2b6d53ae117a2f95fe606ec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2521986


Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824762}