Fix issue where SkAutoSTMalloc doesn't default initialize std::unique_ptr.
Fix issue where SkAutoSTMalloc doesn't default initialize std::unique_ptr.

TBR=jvanverth@google.com

Change-Id: Ifd29f55956f640dfc972490244d5d356bac89759
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/225418
Reviewed-by: Brian Salomon <61fabaaf0876044aeedfd8e351e485c028d69e7b@google.com>
Commit-Queue: Brian Salomon <61fabaaf0876044aeedfd8e351e485c028d69e7b@google.com>

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