[LayoutNG] Use legacy layout for the 'text-combine' property

This patch forces legacy layout for the 'text-combine'
property, for the first phase of LayoutNG.

Also extracted the "if" block to a function because it gets
large enough.

Bug: 636993
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I2d67e26c506bc560eaf31c574d8552adb785bfb1
Reviewed-on: https://chromium-review.googlesource.com/1144594
Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577254}
2 files changed