Preserve presence/absence of optional matrices in CanvasKit

With the change to stop eliding Identity local matrices in:
 https://skia-review.googlesource.com/c/skia/+/842376
It becomes important to preserve the presence or absence of the local matrices in CanvasKit.

Change-Id: Ia445e8a63653b64b02ee269b784161846848596c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/845996
Reviewed-by: Kaylee Lubick <kjlubick@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
1 file changed