Parse text-underline-position left/right

Intent to implement:

This CL implements parsing and serialization for the full spec grammar
auto | [ under || [ left | right ] ]

The new values left and right are hidden behind an experimental feature flag, TextUnderlinePositionLeftRight.
(Support for   auto | under   has shipped previously.)

This CL does not include painting support for the new values.

Bug: 313888
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ie1ba7ecb253e8966b0a9d6f0d0b15df310dfa0a1
Reviewed-by: Koji Ishii <>
Reviewed-by: Kent Tamura <>
Commit-Queue: Eric Willigers <>
Cr-Commit-Position: refs/heads/master@{#575546}
14 files changed