[skottie] Relax constant keyframe parsing

Constant keyframes currently requiere an "h" property/marker.

In order to support keyframed text parsing, relax this to consider any
keyframe missing an end value as constant.

TBR=
Change-Id: Idf332dc0174ee0ee6773708518a0bbc284f8860f
Reviewed-on: https://skia-review.googlesource.com/149265
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
1 file changed