Postpone enabling CSS line-height-step on stable

This patch postpones r462778[1] that enabled CSS line-height-step on
stable.

While we do not see web-compat risks, we heard that enabling on stable
gives a pressure to discussions, and that delaying can make it easier.

We expect this feature to grow only slowly. The previous similar feature
was at 0.007% UMA. I hope this property will be used more widely by East
Asians, but it will still take time to grow. Delaying by a few months
will not make much differences in the value of the feature.

[1] https://codereview.chromium.org/2797973005

BUG=714956

Review-Url: https://codereview.chromium.org/2833323002
Cr-Commit-Position: refs/heads/master@{#466910}
2 files changed