Fix unused variable warning in runtime/gc/heap.cc. am: 58b1b82dc2

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

Change-Id: I07d46f8b7800a4afcce8fb8af42fc983da0beb82
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>