Make ARM compiler happy after 0ecd9e1b

BUG=
TBR=hpayer@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/1219863003

Cr-Commit-Position: refs/heads/master@{#29454}

src/heap/heap.cc

@@ -927,7 +927,7 @@ bool Heap::CollectGarbage(GarbageCollector collector, const char* gc_reason,
   }
 
   bool next_gc_likely_to_collect_more = false;
-  intptr_t committed_memory_before;
+  intptr_t committed_memory_before = 0;
 
   if (collector == MARK_COMPACTOR) {
     committed_memory_before = CommittedOldGenerationMemory();