[heap] Fix -Wshadow warnings

Bug: v8:12244,v8:12245
Change-Id: I5eb07a60881db655ea70dc83189ed1a0447f0bea
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3278688Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77912}

src/heap/allocation-observer.cc

@@ -60,8 +60,8 @@ void AllocationCounter::RemoveAllocationObserver(AllocationObserver* observer) {
   } else {
     size_t step_size = 0;
 
-    for (AllocationObserverCounter& observer : observers_) {
-      size_t left_in_step = observer.next_counter_ - current_counter_;
+    for (AllocationObserverCounter& observer_counter : observers_) {
+      size_t left_in_step = observer_counter.next_counter_ - current_counter_;
       DCHECK_GT(left_in_step, 0);
       step_size = step_size ? std::min(step_size, left_in_step) : left_in_step;
     }

src/heap/new-spaces.cc

@@ -741,8 +741,9 @@ void NewSpace::Verify(Isolate* isolate) {
 
       if (object.IsExternalString()) {
         ExternalString external_string = ExternalString::cast(object);
-        size_t size = external_string.ExternalPayloadSize();
-        external_space_bytes[ExternalBackingStoreType::kExternalString] += size;
+        size_t string_size = external_string.ExternalPayloadSize();
+        external_space_bytes[ExternalBackingStoreType::kExternalString] +=
+            string_size;
       }
 
       current += size;