Fix unused variable warning in runtime/gc/heap.cc. am: 58b1b82dc2 am: 21ac9cf455 am: f3cfa724f7 am: 1b4056c3fb

Original change: https://android-review.googlesource.com/c/platform/art/+/2223108

Change-Id: I05a587e349393fec67f362186f634024c5052f8a
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>