Implement SkUserTypeface::onOpenExistingStream()
Implement SkUserTypeface::onOpenExistingStream()

Since custom typefaces are not backed by real streams, return nullptr.

Change-Id: I757f35df2629cb997dfae61f8d0caf9ca1510ce4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/563976
Commit-Queue: Ben Wagner <53965154ffeecc4a10886eab9259e95965f2979a@google.com>
Commit-Queue: Florin Malita <a8262bf13ea8c26821e91ea9bca6740a77c96dcd@chromium.org>
Auto-Submit: Florin Malita <a8262bf13ea8c26821e91ea9bca6740a77c96dcd@chromium.org>
Reviewed-by: Ben Wagner <53965154ffeecc4a10886eab9259e95965f2979a@google.com>

google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia