make SkExecutor::GetDefault() a little threadsafe
make SkExecutor::GetDefault() a little threadsafe

We note SetDefault() isn't threadsafe, but don't mention anything about
GetDefault().  So it feels like it ought to be thread safe.

But it's not threadsafe as-is, since it writes to gDefaultExecutor.
With tiny tweaks we can make it instead only read gDefaultExecutor.

Change-Id: If229afda3d0bfb4fe491137fd2fc2226d0eafafa
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/377918
Reviewed-by: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>
Commit-Queue: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>

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