[heap] Use size_t throughout MemoryAllocator

src/heap/gc-tracer.h

Index: src/heap/gc-tracer.h
diff --git a/src/heap/gc-tracer.h b/src/heap/gc-tracer.h
index 21004f60e7b61666f9433d93e75ab9bfa0ea375f..e8c72c1e2c36037167ab2f7d1d21b81fa6545a41 100644
--- a/src/heap/gc-tracer.h
+++ b/src/heap/gc-tracer.h
@@ -221,10 +221,10 @@ class GCTracer {
     intptr_t end_object_size;
 
     // Size of memory allocated from OS set in constructor.
-    intptr_t start_memory_size;
+    size_t start_memory_size;
 
     // Size of memory allocated from OS set in destructor.
-    intptr_t end_memory_size;
+    size_t end_memory_size;
 
     // Total amount of space either wasted or contained in one of free lists
     // before the current GC.