Ship 'left' and 'right' values for 'text-underline-position'

This patch marks these new values as "stable" in
runtime_enabled_features.json5.

Intent to ship thread:
https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/gXXMY1grZ-o/b0y3ENIaCQAJ

Bug: 313888, 854091
Change-Id: I2cb4174e644d338163ad5162c7574aaee75d3abd
Reviewed-on: https://chromium-review.googlesource.com/1226485
Reviewed-by: Kent Tamura <tkent@chromium.org>
Reviewed-by: Eric Willigers <ericwilligers@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591412}
2 files changed