[skottie] Always use explicit text line height

Line height is always specified for JSON text values, there is no need
to treat as optional or have a fallback based on font metrics.

Change-Id: I468666e82dab74203fee985503c020217e0d4db8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/215829
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
3 files changed