Fix skottie2movie gpu async crash
Fix skottie2movie gpu async crash

We need to ensure all async reads are completed before we finish
encoding -- otherwise late reads will fire from the GrContext destructor
and crash because the encoder is no longer valid.

Change-Id: I0065561d2411ddd202897e625a187592e40ced87
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/447184
Commit-Queue: Florin Malita <a8262bf13ea8c26821e91ea9bca6740a77c96dcd@google.com>
Reviewed-by: Brian Salomon <61fabaaf0876044aeedfd8e351e485c028d69e7b@google.com>

google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia