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

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

Change-Id: I596ef4574adb4991606c286cfb62c284971c670f
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>