[LayoutNG] Re-bridge NG text fragment to dom snapshot

crrev.com/c/1175758 bridged text fragments to dom snapshot inspector,
which was however removed in crrev.com/c/1173717 due to a behavior
change.

This patch bridges them again.

Bug: 636993
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: If3d224617defd12d7d2fbc6fcda8a70fff2ede16
Reviewed-on: https://chromium-review.googlesource.com/1231927
Reviewed-by: Emil A Eklund <eae@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592302}
2 files changed