Support for PAlloc's ScopedSafetyChecksExclusion

Provide SK_SCOPED_DISABLE_PARTITION_ALLOC_SAFETY_CHECKS
macro in src/core/SkSafetyChecks.h, which defaults to empty
unless defined by partition alloc settings.

Bug: 534908403
Change-Id: I4989cb3f75c8a6fb0662fcb9e9e58020adc10d93
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1293937
Commit-Queue: Kaylee Lubick <kjlubick@google.com>
Auto-Submit: Gregg Tavares <gman@google.com>
Reviewed-by: Kaylee Lubick <kjlubick@google.com>