Add memsets to the GrBlockAllocator unit tests.
Add memsets to the GrBlockAllocator unit tests.

These will verify that our blocks are actually set up properly--if not,
we'll stomp over a sentinel word and/or trip an ASAN poisoned byte,
causing the test to fail.

Change-Id: I2dcb5b913d00c408f70c71f2660c6ec6017b452c
Bug: skia:10885
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/332260
Reviewed-by: Michael Ludwig <cb1f3dd24de0f0b611b26bc28bec758a551d9b7d@google.com>
Commit-Queue: Michael Ludwig <cb1f3dd24de0f0b611b26bc28bec758a551d9b7d@google.com>
Auto-Submit: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>

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