Revert of Make ARM compiler happy after 0ecd9e1b (patchset #1 id:1 of...

Revert of Make ARM compiler happy after 0ecd9e1b (patchset #1 id:1 of https://codereview.chromium.org/1219863003/)

Reason for revert:
Revert in order to revert 0ecd9e1b

Original issue's description:
> Make ARM compiler happy after 0ecd9e1b
>
> BUG=
> TBR=hpayer@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
>
> Committed: https://crrev.com/bef1cbfdf9991f05ca142448098e8076cb7c2d26
> Cr-Commit-Position: refs/heads/master@{#29454}

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

Review URL: https://codereview.chromium.org/1221303002

Cr-Commit-Position: refs/heads/master@{#29469}

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 = 0;
+  intptr_t committed_memory_before;
 
   if (collector == MARK_COMPACTOR) {
     committed_memory_before = CommittedOldGenerationMemory();